Do you have the quick connects in your hose? It is pretty easy to switch those around & mix up in & out <did that a few times myself!> There are in/out arrows on the top of the Magnum 350, make sure the hoses are connected the right way.
The other thing may be the small up tube inside the filter not lining up with the "out" port on the lid ... I always have to first line that up before closing the lid, or the black tube will be slightly tilted & the pump is not connected to the outlet port. There is also an o-ring at that connection. If it is missing/broken that might be the problem.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.