What cells are you using? Cells can sediment or get trapped. Large cells (about micron) can follow preferencial streamlines in the flow due to inertia (is it possible they are bypassing your region of interest?).

To see cells in a high speed flow you need a high speed camera. Or, if you have a suficient number of cells, long exposition images.

I am using MCF-7 with diameter ~18um and MCF-10A with diameter ~15um. I have tried to record a video when flowing but I cannot see cells either in confining channel what I am interested or bypass channels. I am so confused about it, looking forward your reply. Thanks!

Do you have a high speed camera? If you don't you have to work with very small flowrates to see any particle/cell moving with the flow.

Your cells are larger than the 10 micron you give for the channel. They are so large that they can be trapped or adhered upstream to your region of interest.

Test your system with different, smaller particles. Reduce the velocity. Check your camera. Use particles in high concentration.

Actually, I think the camera I used should be a high speed camera. And apart from that, I used 100Pa for inlet pressure which could be a slow flowing. What I guess is that cells might be blocked at the inlet hole. But I am stil not sure about the problem...

Do you use syringe for inlets? If you use low flow rate, large cells tend to settle down in the syringe. Try higher flow rate or insert a small magnetic bar in the syringe and stir from time to time.

Try to get static flow first, only to ensure whether the cells got inside the channel or not.