please help

Dear lammps users
I want to simulate a process of pulling out a carbon nanotube from nature rubber ,i used the fix move linear command in the following way :

fix endcnt cntend setforce 0.0 0.0 0.0

fix move cntend move linear 0.0 0.0 0.001 units box
actually when i calculate force as mention below the force is fluctuated around zero value !!!

thermo_style custom step temp time density etotal press f_ endcnt [3]

someone can help me ?

Think Newton’s 3rd law and measure the forces from the matrix (rubber).