I have been putting together a new 1911 that I ordered from Fusion. Almost there, except for one small problem that I cannot figure out. Hammer cocked, thumb safety ON.....the grip safety springs back in forth in a free motion as it should. Hammer cocked, thumb safety OFF...grip safety is binding and not springing in and out. What may be causing the binding? The lug on the thumb safety or the ear on the grip safety that engages the trigger bow?