About the error—— lost atoms

Dear all,
I am running an MD simulation with lammps .My simulation has two kinds of potential, lj/smooth/linear and harmonic/shift/cut.But the lj/smooth/linear potential is only effective when the distance between two atoms is less than equilibrium separation.
When I parallelly calculate this model using two cpu, there is no error at any time.However , it presents the error “Lost atoms: original 139249 current 132085”, once I use four cpu.
May I have your suggestions ? Many thanks for your reply .

Search “lost atoms” in the web page, http://lammps.sandia.gov/mail.html, and you will find plenty of discussions on this topic.