The summer pruning is just as essential, and it is best to prune hard. The purpose of the summer prune is to restrict and cut back the vigorous growth, so the plant will throw out new flower shoots in the spring. Viewing the main framework, and where you want the plant to grow, cut back and prune hard the long wippy (soft) growth to about 15cmc (6") Leave unpruned that which is needed for the framework.
You may need to cut off a lot of the long wippy growth and also cut off all laterals which appear at the base of the plant. These are shoots being thrown out at the base and remove all.
Thanks to Helen Yemm writing in her excellent gardening column, who remarked on the staining caused by Wisteria sap. It leaves brown marks on your clothes which nothing will shift. This makes it most important to be wearing your oldest gardening scruffs for this job.
A final small point on the time and maintenance required by Wisteria. It drops a lot of leaves in November and December, see image left, which also takes some maintenance clearing them up.
If after trying everything, pruning twice a year for a year or so, watering and feeding it still will not flower consider abandoning it and buying another. Write it off to experience and when planting your new Wisteria, make sure it is in a sunny, sheltered spot.