Cause 1: Wrong place
The wisteria loves a spot in the sun or half-shadow. Less than six hours of sunshine a day will not make it happy. So the east, west or south side of the garden or facade is fine. If the Wisteria is in the shade for a large part of the day, it will not produce flower buds.

Cause 2: Poor soil
Wisteria likes soil that is rich in humus. So give it extra nutrients. Also make sure that the Wisteria is not too dry. In the full soil, it can take a beating (read: wind and weather), but if it is in a pot, you will need to protect the root ball in a harsh winter.

Cause 3: Not pruned
If you want your wisteria to flower abundantly, you should prune it twice a year. If you do not do this, the plant will become a tangle of thin branches. It will then put all its strength into growing and not flowering. Pruning is done in winter and in summer. Both times you do it differently:

In January or February, the Wisteria is still bare and you can see its shape clearly. You then have to cut back all side branches to two or three buds. Preferably on a frost-free day and with the use of sharp, clean pruning shears, to prevent infectious diseases. Leave the main branch alone, unless the wisteria is really growing too tall.
In July or August prune your wisteria once more. Cut back all side branches to within fifteen centimetres of the main branches.