What makes Brahmas come back to human world at death? If a human dies after obtaining a Jhana(and preserved it) he definitly gets his rebirth at a Brahma world.

So, what makes a Brahma come back to human world upon death? Is it possible that he was unable to preserve the Jhana?
Everything is impermanent. What the Brahma enjoys is the Pala or fruit. The fruit of any Karma including that arising from developing the Jhanas also passes after sometime. If you develop the Jhanas they also have a finite duration which you can have them before you loose them. The duration can vary based on past pratice, but as always finite. As in many cases if you lose the Jhana before the point of death then you will not be able to go the Brahma world as you need to have the Jhanas at the point off death for it to bear fruit.