As you just changed out the bottom manifold and replaced a grid-

1) were the slots that fit into the top manifold there and properly in place?

2) did the fabric of the grids fit tightly up to the top manifold - i.e. did you tighten them down far enough?

3) while it was apart, did you thoroughly clean the grids with a good pressure hose and chemically clean them if the water was slow to drain out of them?

4) did you check all the other grids for rips or tears?

5) have you tried putting the multiport on 'recirc' to see if the skimmer suction and return flow are better?

These are the quick things I would check if i were working on this