Welcome to the forum!
I'll trust that the grids are fine and the multiport gasket is good.
How about the o-ring that seals the manifold to the return pipe? Also, are the 'fins' on the top of the grids whole and fully seated in the manifold, AND the fabric all the way up and, at least, to the manifold?
We're running out of places for DE to get through the system, so I hope it's one of the things I mentioned