Set constant velocity?

Hello all!

I am new to LIGGGHTS and have been running to issues which I imagine will have easy fixes, but I can't seem to figure them out. One such issue is that I have been unable to make my particles move at a constant velocity. I am simulating a packed bed of particles moving past a box of metal, and would like to keep my particles moving at a constant velocity in the y-direction (I have periodic boundaries in the x- and y-directions so that particles that move outside the boundaries of my simulation reappear on the other side). I am using insert/pack to insert my particles, where I am able to set the initial velocity. I also use the velocity command after the first insertion, but for some my reason the particles steadily slow down until they come to a stop after insertion. I have included my insert/park and velocity commands below - any advice would be greatly appreciated!

# particle insertion
fix ins all insert/pack seed 123457 distributiontemplate pdd1 &
insert_every once overlapcheck no vel constant 0.0 5.0 0.0 &
volumefraction_region 0.2 region inlet ntry_mc 10000

# set velocity
velocity all set 0.0 5.0 0.0

Thank you,
Zoe