water it much more often as a grape is nearly all water .

You need to thin the bunches out. There should be a bunch every 6inches. You can also thin out the fruit in each bunch. Remove the central fruit and any that are small or mishapen.

the question doesnt sound like the plants got loads to many grapes on it volunteer but i mite be wrong . what you say is spot on about cutting down on the bunches and amount of grapes per bunch.just out of interest which is Sylvia i wonder . both answers are very relevent when the plants mature . i just get the impression it isnt is all.