Why don't electrons from conduction band jump to the acceptor level? One basic idea of Band theory is that electrons always try to occupy the lower energy states. Acceptor levels have vacant low energy states . Then why do electrons from Conduction band don't fall back into acceptor levels? Why always valance band electrons go to acceptor levels giving holes?
 A: In metals, the valence band is completely filled. So there is no space for more electrons. 
In semiconductors or insulators however to begin with there are no electrons in the conduction band. And the ones that get excited to the conduction band have a probability to de-excite and release a photon. Which is why we have to provide a bias voltage to not let that happen. 
A: I disagree with your assertion that "electrons always try to occupy the lower energy states". The electron occupation number follows a Fermi-Dirac distribution. Yes, this is biased towards lower energies, but it does require that some electrons are at higher energies.
This is an effect of temperature: at a given temperature, the system must have a certain amount of energy. If all the electrons were always in the lowest available states, then the system would always have the exact same amount of energy, and thus it would always have the exact same temperature (zero). This cannot be true; we know that you can change the temperature of a solid, so the electrons cannot always occupy the lowest energy states.
