I have an XP3. I love it. Anyways, what everyone has said is what I'd check, but there's also the othr thing on these filters. If you're using the flow restrictor on the return, make sure it's open all the way. Because if it's not, well, not a lot's gonna flow into it. The other thing, make sure the hose structure has been firmly seated in the top, and make sure the latch is all the way down.
125 gallon saltwater: 55gal sump, 4 Blue-Green Chromis, Purple Tang, True Perc, Firefish, Royal Gramma, 5 Ricordea, Bubble Coral, 15 Pulsing Xenia, Green Star Polyps, Deresa clam, Green-Tip Torch Coral (about 11 "heads"!), Orange Montipora Cap, Purple M. Digitata, Green Slimer Acro. Yongei, 3 Orange M. Digitata, Pink&Green Acro. Millepora