I planted 3 grape vines 3 years ago and this is the first year I am
getting grapes. I'm so excited! Now the grapes are about half the size
of a marble and half of them have fallen off the bunches already. Is
this a sign of my vines lacking something or getting too much of something?

The Concord vines are covered with bird netting just to keep those
suckers in check and the pH of the soil is around 6.0 - just checked today.

Any advice would be great.
Thanks,
Quin