What is the problem exactly? Water is stuck under the plate? Is the rigid plate allowed to move or is it fully constrained? Do you have constraints on the rigid plate using *MAT_020 CMO and CON1 and CON2 parameters? If the rigid plate is fully constrained, then you won't have motion of the plate and the water will be stuck under the plate.
Also, what is the mechanical timestep vs the fluid timestep? Make sure the Mechanical timestep is smaller than the fluid timestep; this way, the fluid timestep will be equal to the mechanical timestep.
You can try changing the timestep and see the effect.
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