But you said they should move apart. Perhaps they are in the process of coming together to form one lump?
Apologies, I've misunderstood your question. They will eventually move apart from each other until they are evenly distributed around the orbit they're in - they may well break up from the impacts, too - but eventually they'll all fall (independently) into the planet.. Much like Saturn's rings, which are made up of a host of small fragments bouncing off each other.
Given the size of them individually, it's unlikely they have enough mass to draw each other together - if they had, it's unlikely they'd be in that stable and small an orbit.
O.
