You've got a sizing problem.
Locally, I've worked with large commercial pools for 25+ years. On a fairly heavily used commercial pool, you need a feed system capable of delivering at least 10 ppm of chlorine per day. On a 100,000 gallon pool using trichlor, that translates into 9 1/4 pounds of trichlor per day. Most days, you should use 1/3 to 1/2 of that. But, you'll need that 10ppm / day feed rate to keep up.
Installed out of the box, each RB320 can feed 0.67 lbs/day or 1.4 lbs/day combined. That's about 1 ppm per day -- not even enough to keep up on low load days. If you re-install with a top feed (long tube from box!), you'll score a capacity of 2.5 lbs/day, which is enough to keep up under ideal conditions. Of course, at that feed rate, you'll need to recharge the RB320's daily, since the feed rate diminishes as the number of tabs in the tube drops.
Of course, this assumes that the RB320 tees were installed correctly. They are directional, and I have often seen them installed backwards -- which leads directly to the "no flow" situation you describe.
By comparison, the HC3330 you replace stored 30# of trichlor (vs 4.5!) and has an individual feed rate of 4.5 lbs / day which is 80% higher than the maximum COMBINED output of the (2) RB320s.
http://www.pentairpool.com/pdfs/rainbow300DS.pdf
http://www.pentairpool.com/products/...eeders-140.htm
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