We had about two of them who had that type of problem, turns out the magazine catch had worn down and the mag was sitting about a smidgen too low and the hammer was just glancing off of the valve, when they pressed up on the magazine it fired normally.
http://www.marstar.ca/AssemblyColt1911.htm
Here's a guide to dissasembling 1911 guns. It works, even if it isn't real steel. Watch out for step number 9, the part that presses on the safety will shoot out if you aren't careful.