I use the 5 gallon Rubbermaid MTL with Stainless False bottom. I consistently got stuck sparges. Now, I just use a large Nylon Grain Bag inside the cooler and haven't had another stuck sparge. This info might be helpful?
I have had the weight push the false bottom down so the spigot was pushed to the floor of the cooler causing a stuck sparge. Try unscrewing the 'L' a turn or two.