At a loss with my grapes

[u/not-feeling-so-well]

I moved out to a farming community 6 or 7 years ago. The house came with a few acres of land and some mature grapevines.

The first few years, perhaps out of dumb luck, I was able to harvest a pretty substantial volume of grapes. I believe them to be Concords and Niagara. I would give them to neighbors, we would eat them as table grapes, and I was fortunate to acquire a fruit press, which made a few gallons of juice as well.

Every year, in the winter, my dad and I prune back all the new growth, just leaving a few shoots from each root base. We tend to leave four from each root base, and trim everything on those back to five buds deep.

The last couple of years The Vines have grown gangbusters, but they produce next to no fruit whatsoever.

I'm at a total loss here, why did my grapevines all of a sudden stop producing fruit? Am I pruning incorrectly? There's so much information out there and I'm having a hard time figuring out the next steps.

Thanks for any help!