I don't understand why you're having this problem, but I do 50%-75% weekly and never lose prime, with it running OR unplugged (depends on mymood). I will admit however that adding some teflon tape between the intake tubes coupling areas (where the intake pipes/extension-meet/join) was second nature to me when i installed it. Maybe everyone does'nt think the way I do...dunno?
Also, I have 5 of these (3 Aquatops/2 sunsun) as well as 1-FX5 and 1-FX6, never a problem with any of them. I think some perople make things harder than they have to be, for instance cleaning a canister filter by weekly/ monthly is rediculous to me. Even Eheim does'nt recommend that often!
In MY opinion, If you are losing that much flow you are either using those fine polishing pads, or are SERIOUSLY OVERSTOCKED. I NEVER open my can(setup as BIO ONLY) more than every 90 days, and I've never lost more than 25-30% flow I estimate. I do however use prefilter sponges on ALL my intakes, as well as using HOB's for mechanical because they are easier to clean monthly. Nitrates hover at 20-40ppm with 50% waterchanges weekly...
This has just been MY experiences the last 28 years(I still have my first canister - Magnum 330), yours may vary....