This is either a pruning problem or you have a seed raised plant, not a grafted one. Prune long shoots (unless needed to extend the spread of the plant) during summer, back to 6 leaves and then again in winter back to 2 leaf buds. This will build up flowering spurs. Keep the plant to a size that you can manage, by this method of pruning.

Follow the instructions for pruning that Volunteer has given, but it also helps if you restrict the main stem to 1 or 2 stems only, then when it gets to the height you want (it may already be there, I don't know how long you've had it) chop the top of the stem/s off. Pruning from then on is to cut back the laterals (side shoots) as instructed above. Remove any growth coming up from the base, or lower down the main stems. Bear in mind that most have to be 7 years old before they flower.