Three possible solutions, and I have personally made use of all of them with great success.
1. A common reason why the flow of water through the Pentair 320 canister is insufficient to dissolve the chlorine tablets is that the pool filter is dirty or just too old with ingrained sediment. If your pump pressure is continuously running high then this usually indicates a dirty or old filter that is making it difficult for the pump to get an adequate flow of water through the chlorinator. If thoroughly cleaning it makes no difference, treat yourself to a new filter; it's often an immediate fix and the pump pressure drop may amaze you. Old filters that have served their time are deceiving. They may look clean but the fine sediment trapped in their fibers can seriously degrade performance downstream in the chlorinator.
2. Another trick to help SEE what is going on is to replace the short black feeder hose to the Pentair 320 canister with food grade silicone tube, 5/8" outside diameter, 1/2" inside diameter, and a 4" long piece of 1/2" diam Pyrex tubing inserted into the middle of the silicone tube to serve as a clear sight glass. Silicone tube is rated 500 degF and is available on eBay. Pyrex tube with an outside diameter of 1/2 inch (13 mm) can be bought cut to length for about a dollar per inch from eBay sellers. I also strongly recommend swapping this new feed tube assembly to the UPPER inlet of the chlorinator and moving the plug from there down to the lower inlet. See attached photos.
You will need a 9 inch and 4 inch length of silicone tube, and a 4 inch length of Pyrex tube. Insert the ends of the Pyrex tube 1/2 inch into each piece of silicone tube with some plumbing PTFE Teflon paste smeared all around the outside of the Pyrex ends. Do not get any paste inside the Pyrex tube, and do this rotating the Pyrex as you insert it into the silicone tube to ensure that it forms a complete seal. Then install a pair of 5/8" automotive hose spring clamps over both joints, so 4 clamps in all. I used Corbin clamps number 705-1303 from Napa auto supply, installed so the crimp tabs are rotated to be on opposite sides of the tube.
Now you have a new feed tube 16 inches long in total, 3 inches of which is the Pyrex sight glass. Just take the unions and spring off the original black hose and transfer them to the new feed tube, putting the spring on the longer piece of silicone tube. The 4 inch piece of silicone tube should be at the top of the feed where it enters the Pentair control valve.
You will now be able to observe the rate of flow of bubbles in the Pyrex tube (transparent silicone tube may become opaque over time because of the chlorine but Pyrex remains clear) and know if water is proceeding up the tube and then cascading down over the chlorine tablets. Adjust the Pentair control valve to check if the bubbles speed up and slow down.
3. Lastly, when you open the Pentair 320 to insert new chlorine tablets, the water will normally drain down and out of the canister. IMPORTANT - When you restart the pool pump, keep the cap OFF the canister but have it ready. Assuming you have the Pentair control valve set above 0 (zero) flow, you should soon see water starting to flow through your new PVC feeder tube and then entering and rising up in the canister. When it gets past the top of the chlorine tablets screw the cap on until it feels firm but not cranked hard. If it leaks a little, tighten until that stops. Now you have got rid of that potential air trap that can prevent water rising to completely cover the tablets. Do this EVERY TIME you add new tablets to the canister.