Rhubarb likes plenty of manure/compost as well as water. Dig out some of the soil with a trowel and add blood & bone, pelletised manure, etc. this should help to give it a push along. Plants growing in pots are entirely dependant on what we provide for them so adding some extra nutrients will help. Make this one of your winter jobs when the plants are more or less dormant and you can divide the plants, if needed, at the same time. Rhubarb is a very productive plant and if you sweeten it to taste with stevia (a natural non calorie sweetener) after it is cooked you get all the flavour minus the calories. Trust this helps.

- John