Riser tubes vary but......
Check both top and bottom strainers (the things with very fine slits in them) These can get blocked with resin beads- use a tooth brush to clear. Also, some vessels have a mesh filter inside the riser tube itself, you may also want to check that too. I believe that many people have to have their pump pressure turned up far beyond what should be required as a direct result of the above. I clear my DI internal strainers at every resin change and I get my required flow with 100m of 6mm hose with a flow setting of 2 and pressure setting of 7 on a MK-I digital Varistream. A properly maintained DI vessel (something which seems to be overlooked too often) should offer virtually ZERO resistance to water flow.