Probably the hand. Does it cycle and lock up properly when cocked at normal speed with rounds in chamber? A slight lag in lock up is somewhat common but still means it is out of time and is going to need repair. It may function for quite a while if it locks up at normal cocking speed but only shows the problem when you are trying to make the bolt drop late. Repair is fairly easy for this problem and it is considered normal maintenance for this type of action. The hand can be stretched which is the typical repair for this problem and the Jerry Kuhnhausen book, Colt Double action Revolvers a shop manual ,Volume l gives detailed instructions for you to follow or for a local gunsmith.