It's a pain, but I would remove all media and baskets, button it up and see if it primes and runs without the baskets in place.
The primer assembly might have media caught in it? FWIW, I've got a Marineland C-360 canister. I've had it going for almost 5yrs now. I never use the primer button. I fill it maybe ~90% with water before I button it back up and run it.