Such many deficient mags is hard to beleive. I would check the mainspring. In 5" 1911's 18½ lbs. springs are usually installed by many gun builders and they are too strong IMHO, making the gun more difficult to shoot. Get and try a 15 lbs. mainspring.
Do you mean a 15 pound RECOIL spring? The factory MAINSPRING is 23 pounds so a 15 pound mainspring would definitely not give reliable ignition.