Well, you might be overwatering. Keep the soil damp but not wet. Air has to get to the roots too. If you're not over-watering then maybe a little nitrogen fertilizer will help. Blood meal scratched into the soil will give 'em a boost and still be considered organic. Another good fert is fish/seaweed emulsion. You really haven't given enough information on your growing conditions for anyone to give you promising info. Hang in there and keep reading other posts, there's a ton of good info here. Best of luck.

I too would put my money on too much water. it's a common mistake with plants in containers - especially a problem for tomatoes. Try letting them dry out more between waterings and check moisture deep in the soil before watering.

Dave