Hello, we had the same problem while doing immunofluorescence but in P6 plates. We found out that using PBS suplemented with Ca2+ and Mg2+ cell detachment was reduced a lot. Maybe you can give it a try.
I agree with Aday González Additionally, try to be as gentle as possible while carrying out washing. Pipette down the side of the well and off the well without disturbing the cells. Also, avoid shaking the plate while performing the washing step.