particles begin to vanish from CFD part

Dear colleagues
I have developed a new CFDEM solver, when coupling starts particles begin to vanish from CFD part, I mean Lagrangian particles, that could be loaded to Paraview from particle cloud. When the run ends up, more than half of particles are lost. But from DEM part particles could be seen and there is no problem, no DEM particle have left the domain. What may be the cause of this phenomenon? Any help is appreciated.
Talat,