I find that once my frags start encrusting onto the rack its time to move them to an area of the rack with slightly less flow. Generally sps will encrust and thicken if they are in really heavy flow areas so they have a better support structure. I have a 34" long frag rack in my setup at the moment, and at the end of the rack (nearest the powerheads) I tend to place the thinner, more spindly frags. They tend to encrust and thicken at the main branch for several weeks/months before starting to take off. I then move those towards the center of the rack (more light, less direct flow) and they usually start sending up branches from the base at that point. If I do get a stubborn frag that keeps encrusting onto the rack, I let it go a little bit, and then break off the encrustation (?) and mount it onto another frag after moving the original culprit. Ive had good success with moving the frags around on the rack this way. With some montis and other encrusting corals (psammacora and leptos) I place them on a small tile (3x3 is the measurements I think) and then they get moved off the rack (usually onto the sand) at that point.
Might be difficult to move the frags if you dont have a good sized frag rack, but thats how I deal with the issue.