Check to see if the inlet screen at the intake of pump is clogged with debri.
Second, if the dishwasher discharges into an Air Gap, take the trim piece off at the flood level rim of sink and inspect it for debri.
If installed into a tapped tailpiece or direct connect to a garbage disposal, disconnect dishwasher line and run a cycle into a 5 gallon bucket and see if water remains after cycling. If so and all debri has been removed, it might possibly be a constriction in the line where it connects to dishwasher or the line is clogged from years of use.
You will have to try these to see what resolves the problem.