particles cross wall

Hi, Christoph, i simulated a compression test, but i found the particles crossed the top wall after compression. Hete is the simulation procedure. First particles are poured into a 0.5 m high cylinder without topwall, result is saved for restart; then in the restart file, the topwall is imported and moved to compress the particles. I couldn't find the reason for particle crossing wall. Can you help me? by the way the version i used is 1.2.6 beta.
Thanks,
Ada