[lammps-users] weird problems about lammps

I think I found something weird when lammps runs with “fix aveforce 0.0 0.0 0.0”.I don’t know why.I think I need your help.
There are water molecules between two graphenes. The bottom one is fixed,and the top one is free,the x,y,z boundaries are periodic. The width of Simulation BOX is larger than the distance betwwen top and bottom graphene.
But with the following sentence in lammps in-file, the result is that the top graphene is static at all. I’m sure it should move along z axis in physics. But it shoud be free along z axis because I just fix the top graphene with “aveforce 0.0 0.0 0.0”.

The example/flow and example/obstacle input scripts
use the fix aveforce command, so I'd look at those to see
if you are using it correctly.