M-41 Fail to Fire

I’m working on a M-41, mid-70’s manufacture, that fails to fire most of the time. I am in the middle of troubleshooting and thought I’d solicit some advice. Apparently the hammer is dropping to half cock rather than all the way when fully assembled. With the slide off, the hammer drops properly. First thing I did was get the overtravel screw out of the way so that’s not an issue. I am getting ready to swap the slide with one off a known reliable shooter. 
Any other advice?

Take off the right side grip.

First check the disconnector groove on the bottom of the slide to make sure there's no gunk in there keeping the disconnector tab pushed down a little.

Second check the disconnector tab on the trigger bar to see if its worn or bent or has a burr that keeps if from fully rising into the disconnector groove or if there is a problem with the trigger return spring.

Third check the magazine disconnector bar is rising up enough to allow the trigger bar to fully engage.

Basically anything that can keep the trigger bar from fully engaging the sear tab and therefor reduce the movement of the sear.

CR

Another thing to check is to make sure that the slide is fully forward after cycling a round.  If the slide is not all the way forward, the hammer will hit the bottom of the bolt/block and not hit the firing pin due to the angles.  

You should also remove the bolt/block and make sure that the firing pin is clean and the correct dimensions.

Thanks for the responses. The problem was a combination of both things mentioned. The bolt face had a burr that kept the slide from closing completely plus the disconnector and the notch in the slide were not properly matched. Apparently parts had been changed out without proper fitting. Headed for the range tomorrow to see if it is fixed.