I had the exact same issue with my old WA 92F. The part you need to look for is called the trigger bar spring, it puts upward pressure on the trigger bar when the safety is off:There's one of two possibilities:
a.) either your pistol is missing it or
b.) The grips help keep it in place so if they don't put enough pressure on it the spring will pop out of place (this happened to mine when I replaced the plastic grips with rubber hogues). This is how it should look like; it's a real one but it should be the same:
http://www.midwayusa.com/product/299...factory-design