WHen you torque a bolt down, the bolt actually stretches to a point. THe more you tighten it, the more it stretches.
Repair manuals will tell you how much to torque them down by, a lot of times, they do this to ensure that you don't over tighten the bolt and cause it to stretch beyond what its meant for.
Im guessing your bolts got tightened too hard too many times and now they are too narrow to hold as well.
That's the other thing.. they get narrower as they stretch. Its not by a lot, but its causes **** like this.