wall/piston & atom going far away

Dear All,
I am using LAMMPS (14 Feb 2014) and have some questions about wall/piston & shock :
I use fix wall/piston to create shock wave and give the piston a range of velocity:5,10,20,50…During the shocking,I also use fix nve to create a independent condition.
fix 1 all nve
fix 2 all wall/piston zlo pos 1.0 0.0 0.0 vel 0.0 0.0 20.0 units box
The box is ~80A80A300A,containing 112000 atoms.
When running,one or two particles will go far away in the shocking direction.
I change the parameters such as ‘delay’ in ‘neigh_modify’ command via using:
neighbor 2.0 bin

neigh_modify delay 1 check yes
Also,I boost ‘one’ and ‘page’ in ‘neigh_modify’ command
Also,I change the size of box.

However, nothing changes.I wonder how to solve the problem?

Thank you very much for your answer

If you slam a solid with a piston (too hard) atoms
can fly away. What do you see if you visualize
the system? What happens if you slam
the solid more gently? Is your timestep sufficiently
small?

Steve