ssgt: I agree with your analysis. Bullseye guns tend to have a light mainspring - probably somewhere in the 12-14 lb range. When you couple that with a slide mounted scope, the lighter recoil spring may not allow your slide to go all the way into battery. It may allow it to go far enough to almost chamber the round, or look like it chambers the round and your sear plunger may be close enough to the sear plunger indent on the bottom of the slide, but not enough to allow the hammer to fall far enough forward to fully impact the firing pin.
DennisB is heading in the right direction by going to a heavier recoil spring, I believe. My choice here would be for DennisB to get a set of 22 lb heavy duty springs from Wolff Springs in PA - those are the ones I carry in all my 45s, and they should help with his problem.
ssgt - I would go the same route and get a heavier set of springs from Wolff, but for your purposes with a bullseye gun I would first go to a 16 lb set (which is normal factory spring rating) and if that doesn't cure your problem then go to an 18 lb set. What you will need to balance out is the power factor of your loads and the recoil spring tension. If you are shooting light target loads, the 16 pounder may be the heaviest that can properly function your gun with those loads. HTH. Mikey.