Hi 1. At the tips of the sticks where the string starts, the string goes dark, kind of greasy looking and eventually frays and breaks.
Tbh I'm not sure if you mean the handle are slipping of or wearing away.
i've found that with mine as well, they suddenly snap sometimes, and that was with 1 day old string, although the string was frayed, and the diabolo rolled up it really fast, causing tons of force - really annoying