The Great Epic of the Bhārata dynasty 12.318.9

12.318.9

yo yamicchedyathākāmaṃ kāmānāṃ tattadāpnuyāt |
yadi syānna parādhīnaṃ puruṣasya kriyāphalam || 9 ||

If the fruits of man's acts were not dependent on other circumstances, then one would obtain whatever object one would desire. Even men of restrained senses, of cleverness, and of intelligence, if destitute of acts, never succeed in earning any fruits. Others, though destitute of intelligence and unendued with accomplishments of any kind, and who are really the lowest of men, are seen, even when they do not long after success, to be crowned with the fruition of all their desires. Some one else, who is always ready to do acts of injury to all creatures, and who is engaged in deceiving all the world, is seen to wallow in happiness. Some one that sits idly, obtains great prosperity; while another, by exerting earnestly, is seen to miss desirable fruits almost within his reach. Do thou ascribe it as one of the faults of man! The vital seed, originating in one's nature from sight of one person, goes to another person. When imparted to the womb, it sometimes produces an embryo and sometimes fails. When sexual congress fails, it resembles a mango tree that puts forth a great many flowers without, however, producing a single fruit. As regards some men who are desirous of having offspring and who, for the fruition of their object, strive heartily (by worshipping diverse deities), they fail to procreate an embryo in the womb.