Particles crossing wall

Hello,
I am a new user of LIGGGHTS and I am simulating a shear cell with:
- upper surface as servo, to apply a force in y direction
- lower surface moving in x direction
- fixed walls in the z direction
- boundary p f f
All the surfaces are imported as STL files.

I have the problem that after the compression of the particles and during the motion, at a certain time some particles disappear because cross the lower wall and are lost from the domain. The same happens even without movement of the lower wall, i.e. simply with a compression.
This is my script, can you help me?!

Thanks a lot
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atom_style sphere
boundary p f f
newton off
units si
communicate single vel yes

region reg block 0 0.01 0 0.01 0.0 0.01
create_box 1 reg
neigh_modify delay 0

fix m1 all property/global youngsModulus peratomtype 1.0e7
fix m2 all property/global poissonsRatio peratomtype 0.25
fix m3 all property/global coefficientRestitution peratomtypepair 1 0.5
fix m4 all property/global coefficientFriction peratomtypepair 1 0.5

pair_style gran model hertz tangential history
pair_coeff * *

fix part all particletemplate/sphere 1 atom_type 1 density constant 1000 radius constant 0.0003
fix distr all particledistribution/discrete 63243 1 part 1.0

fix bottom all mesh/surface/stress file mesh/bottom_triang1mm_sal.stl type 1 scale 0.001 stress on
fix top all mesh/surface/stress/servo file mesh/top_l1.stl type 1 scale 0.001 move 0.0 -0.007 0.0 &
stress on com 0.0 0.0 0.005 axis 0.0 1.0 0.0 ctrlPV force target_val -0.1 vel_max 0.5
fix front all mesh/surface/stress file mesh/lateral_front.stl type 1 scale 0.001 stress on
fix back all mesh/surface/stress file mesh/lateral_back.stl type 1 scale 0.001 stress on
fix geometry all wall/gran model hertz tangential history mesh n_meshes 4 meshes bottom top front back

region insreg block 0 0.01 0.001 0.003 0.0 0.01
group nve_group region insreg
fix ins all insert/pack seed 5330 distributiontemplate distr maxattempt 200 insert_every once &
overlapcheck yes all_in yes vel constant 0.0 0.0 0.0 region insreg particles_in_region 1000

fix integrator all nve/sphere
compute forces all wall/gran/local force_normal force_tangential

timestep 0.0000001

run 5000

dump dmp all custom 5000 post4/dump*.triangular id type type x y z ix iy iz vx vy vz fx fy fz &
omegax omegay omegaz radius
dump dmpstl1 all mesh/stl 5000 post4/bottom*.stl bottom
dump dmpstl2 all mesh/stl 5000 post4/top*.stl top
fix motion all move/mesh mesh bottom linear 0.01 0.0 0.0
fix time all check/timestep/gran 1000 0.2 0.2

run 3000000
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