If the cleat is sliding along the boom, then it's probably not the cleat that's the problem, its the mounting bracket.
I can't recall right now whether the rotator cleat mounts onto the same type of bracket as the outhaul cleat, or whether it just has a nut and washer that sit inside the track. But either way, you're not getting it tight enough either because the threads in the bracket are possibly stripped, or if it's just a nut on the inside, the nut is probably spinning when you tighten the screw. Another thing you can do if it is loosening up is to put a drop of Loctite on the threads. Let the Loctite cure for 24 hours before using.
Another note - if you're using a plastic cleat, you may want to replace it anyway. Those have a tendency to let the line slip through. Aluminum clam cleats are much better.