why have i no grapes

I bought a grape vine [ Black grapes ]
, this is its 3rd year ,last year we had lots of grapes on it............THIS year even more.
Although I like to experiment on seed growing, you would be better growing your grapes from a plant [ garden centre ]This one we have trained onto our old vine, which stopped bearing grapes.

Depending where you live it may not get enough heat and light to produce grapes - are you pruning it every year or just letting it grow? If the latter it will put all its energy into growing leaves and shoots and not bother about producing grapes. Grape vines need serious and vigourous pruning every year!