I would wager improper pruning. Grape vines need to be pruned to produce well.

Cut it all down, except the strongest biggest branch going up from the ground. Cut that one off at the top of the fence height. If it has two branches on either side, one going in each direction, you may keep those, otherwise, this year, prune away everything that grows new except one branch that goes in each direction along the fence. Tie these to the fence loosely. If these branches then send up branches heading back up, let these tertiary branches get a foot long, then cut the ends off.

Imagine, if you will, a giant upward facing rake, that is the shape you are trying to create.

Then make sure it gets sun, and it should fruit.