I would check to see if the grapes look like the right variety, you may have gotten some dud variety and not cabernet. Besides that I don't see a clear answer. Grapes need to be pruned heavily in the winter, you may have too many shoots if you are not doing that. See the grape growing in pots guide below for other ideas of what you may need to be doing.

Scott

Here is a link that might be useful: grapes in pots