It could be that there's air in the canister. If the spraybar is under water, that's probably what it is. If you look at the canister, can you see a water line? I've had to crack open one of the latches and let the air get out (you'll probably get a little water leakage as you do this). If that's not the issue, I'd check your fittings going in to the canister and make sure it's not sucking air.
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