Yes, I have a Magnum 350 & it is a PITA to get working right ....
In addition to checking for clogs in the tubes, both inlet & outlet, (disassemble the whole thing & run a flexible brush through all the tubes), also check:
1. Any debris in the "sump" thing where the impeller sits. I had some sand in there once & that gummed up the impeller so there was no flow .... Also check for hard water deposits in the impeller magnet or the housing. <I had to soak the whole area with CLR every few months or the impeller will gum up.>
2. Is the impeller broken?
3. Is the output tube (inside the canister) lined up with the outlet hole in the lid? Is the o-ring worn? A missing or worn o-ring at that connection will drop the flow to nothing.
4. If you are running the water polishing cartridge, you need to clean that out every few days .... I can never get the Magnum to run for more than 2 or 3 days using the water polisher ....
5. Also check for airlock in the system ... the filter needs to be manually primed (my manual said to burp the canister to get rid of air ....) Also, loose o-rings may cause airlock & decrease the flow.
Anyways, the Magnum is so high maintenance that my sump is simplicity in comparison ...