about the force controlled boundaries

Hello,

I did a simulation using force controlled doundaries. when i try to unfix it, then i got the erro message as follows:

ERROR: Fix mesh/surface/stress/servo (id zcover_1): illegal unfix command, may not unfix a mesh while a fix move is applied.Unfix the fix move/mesh first (fix_mesh.cpp:242)

but the problem is that i did not use fix move/mesh, why am i not allowed to unfix the "fix mesh/surface/stress/servo"?

In addition, i am trying to define three forces on the force-controlled boundaries during one simulation, i tried to use loopa, variables, unfix (as previous described) ect, but all of them done work. does anyone know how to do that?

Thanks a lot!
Xuetao