Hi, sweetcorn woul be better grown in the ground, as they use up a lot of nutrients, it sounds as though they have already used up the nutrients in the containers, as the leaves are turning yellow, the stems should be thick , and strong enough to hold itself up, and should not need staking, if they aren't, then you're not going to get many corn cobs off them, they're too weak, I would transplant them into the veg patch asap, trying not to disturb the roots, give them a feed of general purpose fertilizer, and keep them well watered, Derek.