How can I fix ADP4504 whirlpool dishwasher?

I have a problem with my ADP 4504 Whirlpool dishwasher, The tablet does not dissolve, the dishes are still dirty after the hottest cycle, I left a cup up the right way in the top pull out tray, it had no water in it, previously it would have had at least an inch after a full cycle. I have replaced the water inlet spray tube (plastic piece that runs down back of machine(internally) from top to bottom) as the seals on the middle section were perished but still seem to have a water pressure problem, ie: getting water to middle spray arm and top spray Anyone any ideas? I have cleaned the machine with a recommended cleaner and that all went away bar a trace of blue in the bottom, I have also checked spray arms for blocked holes, all clear..