How to solve the problem of inexplicable reduction in the number of particles using the coarse-grained particle(CGP)?

Dear researchers:
Recently a simulation of particles behavior in a fluidized bed was carried out using the coarse-grained particle(CGP), with the original particles having a diameter of 100 µm, set in the simulation to 0.002 m. In the initial conditions of the simulation, 212,221 particles were generated, but it was found that the number of particles decreased as the simulation proceeded, and the total number of particles was only 211,440 when the simulation was carried out up to 0.30 s. The particles were then not flow out of the reactor, have you encountered this situation? Is there any special setting needed? Thanks for your help!

Maybe the particles are too soft and going through the wall. Please try increasing the spring stiffness to see if this helps.