sergio,

Are you sure you've set up your power distribution correctly?  Double check of course that the DISCH indicator to the left of the battery button is not illuminated.

Both Bus Ties to AUTO.
Both Utility Buses ON.
Both engine generators to ON
Both gen drives DO NOT TOUCH. :)  

If you've ever pressed a Gen Drive button (under the covers) and then saved a flight, it could be that the gen drive disconnect flag has been saved there, and if that saved flight is your default, it could be your gen drives are actually in disconnected status.

If you're getting this issue and the DISCH indicator is NOT lit, then I can't say I know the problem.

It is a little bizarre.  If you've ever saved a flight tagged as "Default" with the CS757, try deleting that flight and reboot the sim and see if that clears the issue.   8-)