You did backwash, didn't you? Can you backwash now or is it all jammed up?

It sounds like all you probably REALLY need is a new multivalve, as long as the "laterals" in your sand filter are OK. It also sounds like you are sized correctly (re, pump to filter, but your pump is really probably too large, so the 300# filter is a good idea to compensate), just possibly plumbed wrong or have damage to the multivalve.

But, before you just throw money at the problem, put the valve to "circulate", not filter, and see if you get strong suction at the skimmer and a strong flow at the return. If you do, your problem is in the filter or multivalve, but NOT in the pump or the lines.

If not, it may STILL be in the multivalve and you'll have to find a way to cut the valve out and see if the pump is actually pumping, like connecting the return line directly to the pump. If that still doesn't work then, it's not your filter or multivalve, but in the pump or the lines. You can even just disconnect the pump from the filter and turn the pump on for a few seconds--it should be like a fire hose! (I have quick-connects everywhere so it's easy for me to do)

You have to figure out how to isolate each possible source of the problem and show it's working, then move on to the next.

if your filter has the correct amount of sand and hasn't been moved, the odds are, yet again, the problem is in the multivalve. Take it out and look for obvious signs of damage-mangled spider gasket, broken chamber walls, etc.

A new multivalve is a lot cheaper and easier to set up than a whole new filter. Plus, just because your pump is new doesn't mean you don't have to prove it's working. You do, or replacing the filter/multivalve may not fix it.

A little detective work....

Carl