Particles accumulate in cell when using CFD-DEM approach

Hi everyone, when i simulated a 2D fluidized bed by CFD-DEM approach, an abnormal phenomenon occurred that particles accumulated in some cells.
I am not sure whether it is due to the opening DEM_explicitly_coupled or perhaps the other reasons.
Thanks for anybody can give me the answers.

Try to refine the mesh.

thanks for generous reply, i will try to minish the cell size.