Fns plus vertical grid de filter plumbing diagram from pump

If you have the manifold upside down on the ground, the small grid fits into the second hole to the right of the big standpipe hole. That is the reason for the smaller grid, to allow room for the stand pipe.

The FNS PLUS was originally built by American Products under a different name and was absorbed into Pentair's line, recently replacing the PAC-FAB FNS (fiberglass nautilus) also absorbed by Pentair. The two filters have no common background.
The FNS PLUS has been cursed at by more pool techs than I can count. All I can say is practice helps but these are always a pain to get right. Make sure that the narrower of the elements is in the correct spot, the rest do not matter. Also check the manifold on all sides for cracks and make sure the air relief screen is set properly. I wish I had an easy solution for you, but this filter is just a pain, even for the guys that work on them every day.

Make sure that the space in the spreader is aligned with the big hole in the manifold, and that the small grid is 2 spaces right of the big hole. I curse like a sailor almost every time I work on these (FNS Plus), even after 18 years in the business. Your local pool shop should be able to do this for you at no charge.

did you put the grids back correctly and get ti tight also make sure the manifold isnt cracked any leak will let DE back in

COULD BE TOO MUCH CHLORINE.

If all your grids are in good shape you have to look at your manifold real close for cracks and around the internal air bleed. When checking your grids be sure the nipples that go into the manifold are also in good shape and when reassembling make sure all fit comfortably and get them tight. Hope this helps, good luck.