it is quite difficult to discuss these matters in such an abstract way.
fix move is the feature in LAMMPS intended for moving particles on a prescribed path. but using fix setforce 0.0 0.0 0.0 properly after assigning a velocity will also result in a constant velocity motion.
beyond that, it is difficult to give advice on something that one cannot see or reproduce. you have to check your input carefully, watch out for warnings and try to understand what they represent and whether they need to be addressed (most do need some action, but which ones depends on the details of the simulation).
also, it is not clear what you are actually asking here outside of the obvious which you have effectively answered yourself already.
axel.