I wouldn't think that there is nothing on your fishes for the shrimp, because like you said why would they go near the cleaner? Most of the time whatever can be picked by cleaner shrimp is not visible to human eyes, so when you see nothing doesn't mean there is nothing.
What I have learned is the size of the fish and the size of the shrimp matter towards the relationship between your cleaner and your shrimp. My guess is your fishes are bigger than your cleaner shrimp. Consequently your shrimps are a little intimidated by the size of your fishes. That's what happened to me. Once I had a big shrimp with small fish, one always run after the other. I also had one smaller shrimp and bigger fish, and of course the shrimp hides from the fish. That might be the case for you.
All you can do is just wait until your cleaner shrimps grow bigger. Once they have grown bigger and more used to their environment, your fishes will have a pair of nice doctors
, and ocassional alive food (the eggs of your cleaner shrimp).
Good luck and I hope that helps.
stupid_naso