I've had one of those 'monsters' going for over 14 years now and the only things I've replaced are the O-rings. Seems to work just fine for me. Granted I only use it when I need to jack up the CYA and when I'm away for a long time but I still use it. Never had a plugging problem either. A couple thoughts though....when you screw the lid on it is a pressure seal and if there isn't much pressure in the return pipe where the chlorinator is located, due to not much flow resistance between the unit and the return jets, the water level may not climb up high. Try removing the lid, close the needle valve, turn on the pump and wait for the water level to get to within a couple inches of the top then put the lid on. Turn off the pump, tighten the lid, no need to really make it tight as the O-ring is the seal, set the needle valve where you want it and just use it normally. This gives you a full charge of water in the canister. These units have a restriction in the pipe and rely on the pressure differential generated by the flow to cause water to circulate through the canister. If you have a 2 speed pump on low speed or just a low flow for any reason then it may not work right.
Hope this helps.
Al