te taḿ bhuktvā svarga-lokaḿ viśālaḿ
kṣīṇe puṇye martya-lokaḿ viśanti
evaḿ trayī-dharmam anuprapannā
gatāgataḿ kāma-kāmā labhante

Translation of Bhagavad Gita 9.21

When they have thus enjoyed vast heavenly sense pleasure and the results of their pious activities are exhausted, they return to this mortal planet again. Thus those who seek sense enjoyment by adhering to the principles of the three Vedas achieve only repeated birth and death.

Commentary by Sri A.C. Bhaktivedanta Swami Prabhupada of Gaudiya Sampradaya:

One who is promoted to the higher planetary systems enjoys a longer duration of life and better facilities for sense enjoyment, yet one is not allowed to stay there forever. One is again sent back to this earth upon finishing the resultant fruits of pious activities. He who has not attained perfection of knowledge, as indicated in the Vedanta-sutra (janmady asya yatah), or, in other words, he who fails to understand Krishna, the cause of all causes, becomes baffled about achieving the ultimate goal of life and is thus subjected to the routine of being promoted to the higher planets and then again coming down, as if situated on a Ferris wheel which sometimes goes up and sometimes comes down. The purport is that instead of being elevated to the spiritual world, from which there is no longer any possibility of coming down, one simply revolves in the cycle of birth and death on higher and lower planetary systems. One should better take to the spiritual world to enjoy an eternal life full of bliss and knowledge and never return to this miserable material existence.

Commentary by Sri Vishvanatha Chakravarthi Thakur of Gaudiya Sampradaya:

Being filled with desires for enjoyment, they take repeated birth and death (gatagatam).

Commentary by Sri Ramanuja of Sri Sampradaya:

9.21 After enjoying the spacious world of heaven, they return to the world of mortals when the meritorious Karma forming the cause of that experience is exhausted. Thus, lacking in the knowledge established in the Vedanta and desiring only the attainment of heaven etc., they who follow the teaching of the three Vedas on sacrificial rites, come and go. After enjoying the trifling and transient pleasures of heaven, they return to Samsara again and again. But the great souls meditating on Me, who am incomparably dear to them, attain boundless and unsurpassed bliss and do not return to Samsara. Sri Krsna desribes their distinguishing features:

Commentary by Sri Sridhara Swami of Rudra Sampradaya:

Those jivas or living entities reach Swargaloka the heavenly regions due to following the rituals prescribed in the Vedas of karma kanda or actions for rewards. After having enjoyed celestial delights extensively in the form of exquisite pleasure both physical and subtle for a long duration when their stock of merits becomes is finished they again descend to the world of mortals subject to samsara the perpetual cycle of birth and death. With the samsara from the previous birth fresh upon them they take birth in a family of performers of Vedic rituals where desirous of enjoying the delights of heaven they begin the procedure all over again and thus they come and go, back and forth lifetime after lifetime unceasingly.

Commentary by Sri Madhvacharya of Brahma Sampradaya:

This verse clarifies in no uncertain terms that pandering to the demigods and canvassing of lesser gods have serious defects and limitations. Whereas worship and propitiation of the Supreme Lord Krishna is superior and the results are eternal.

Commentary by Sri Keshava Kashmiri of Kumara Sampradaya:

What happens after one has fully enjoyed heavenly delights exhausting all their merit? They must take birth again in the worlds of mortals and begin accumulating sufficient merit all over again by performing the same rituals prescribed in the Vedas for entering the celestial spheres to enjoy heavenly delights all due to motivation of desires. Again and again they must be imprisoned inside a mothers womb for nine months, lifetime after lifetime revolving in this cycle unceasingly because they hanker so much for heavenly pleasures. But when their merit is used up again they are in transition down to the cycle of birth and death and then back to heaven incessantly repeating the same process over and over.

Commentary by Sri Adi Shankaracharya of Advaita Sampradaya:

9.21 Bhuktva, after having enjoyed; tam, that: visalam, vast; svargalokam, heavenly world; te, they; visanti, enter into; this martyalokam, human world; ksine, on the exhaustion; of their punye, merit. Evam, thus, indeed; anuprapannah, those who follow in the manner described; trai-dharmyam, [A variant reading is trayi-dharmam.-Tr.] the rites and duties prescribed in the three Vedas-merely the Vedic rites and duties; and are kama-kamah, desirous of pleasures; labhante, attain; only gata-agatam, the state of going and returning, but never that of independence. This is the meaning.

Commentary by Sri Abhinavagupta of Kaula Tantra Sampradaya:

9.20-21 Traividyah etc. Te tam etc. Of course, they worship Me (Vasudeva) alone. However, the action [like sacrifice] is limited (or is known [to them]) by their aspiration for heaven only. Hence, on account of the weakness is their own being (sattva), they condition the action solely by the result of the heaven. That is why their religious act leads to rebirth and thus they attain the state of going and coming. But [on that account] it is not the inherent nature of the sacrifice to beget rebirth. For instance :

Sanskrit Shloka Without Transliteration Marks:

te tam bhuktva svarga-lokam visalam
ksine punye martya-lokam visanti
evam trayi-dharmam anuprapanna
gatagatam kama-kama labhante

Sanskrit to English Word for Word Meanings:

te — they; tam — that; bhuktvā — having enjoyed; svarga-lokam — heaven; viśālam — vast; kṣīṇe — being exhausted; puṇye — the results of their pious activities; martya-lokam — to the mortal earth; viśanti — fall down; evam — thus; trayī — of the three Vedas; dharmam — doctrines; anuprapannāḥ — following; gata-āgatam — death and birth; kāma-kāmāḥ — desiring sense enjoyments; labhante — attain.