Well, assuming there's no air in the hoses, make sure the outlet isn't blocked from inside the canister. Remember that stuffing the canister will definitely reduce flow. When you take the hose assembly off, check the little clear plastic flapper and make sure it moves freely. Make sure the intake isn't blocked - is there any sort of filter (other than the stock one) on it? Make sure there are no kinks in the line. Do you inject CO2 into the canister? Before it? Is the black plastic lever on the hose assembly all the way down? Is there any gunk in the spraybar, especially at the 90 degree turn just before it (remove the spraybar and see if there's a sudden flow increase). Do you have an inline heater, and if so is it installed in the correct direction?
That's all I can think of atm.