I'm having trouble tracking down the problem causing my flow rate to be low and hope someone here can offer advice.

I have an inground pool with sand filter. With my multi-port valve set to filter or recirculate the flow is low, when set to backwash or waste the flow increases to normal. This seems to indicate that there is a blockage somewhere on the pressure side. Before I try to find a blockage from pump to return jets (which will be difficult as there is no easy access i.e. I will need to cut pipes) can anyone think of anything else that could cause the flow rate to drop so much between recirculate and backwash?

Thanks for any advice.