Grape vine not flowering

Hi.

I've got two grape vines. One a Booksop Glory and the other a Muscadet. Both are on south facing walls 20yds apart. The Booksop is going great and I have to prune off lots of bunches once the flowers have done their bit every year. The Muscadet grew well and gave us a decent crop of grapes for the first few years, though its not flowered for the last 3 years. I cut it back hard last year thinking that this might encourage it. Nothing doing. Any ideas please?