Originally Posted by WHSmithIV
A recoil spring that's a little weak shouldn't prevent the hammer from fully cocking though.
The sear, hammer notch or sear spring are the most likely candidates. Don't fire it a lot until you fix it. The half cock hammer notch won't take a lot of abuse. My bet is your sear spring should be replaced.
The above advice is correct. You may experiment by increasing the tension on the current sear spring finger. The laws of physics dictate that when the slide comes fully out of battery, it WILL drive the hammer back to the full cock position. Since the spent case is ejecting, we know there is no problem with the recoil spring or extractor. The hammer is "bouncing" out of the full cock notch because the sear does not have enough tension to stay in a repeatable position under recoil.
You can fix this problem in about ten minutes. All traditional 1911 platforms are susceptible to this failure.
You can also check this by releasing the slide on an empty chamber to see if the hammer follows. BE SURE TO DEPRESS THE TRIGGER AND HOLD IT PRIOR TO RELEASING THE SLIDE STOP. This will prevent your hammer and sear from being "peened". The hammer should not drop or go to half cock.