Could be a few things but it might be your hammer is chattering and not hitting your bolt hard enough. You might try putting some dry lube in the area of the hammer. Some guys say no, but it works for me.
Once I was even so bold as to put a couple of drops of silicon lube on the hammer.
Lubing the hammer some agree with, others do not. If you use dry lube it will help if that is the cause for your problem.