Think maybe your over fertilising and the plant is using this extra energy boost to make greenery at the expense of flowers. Where have you got the vine situated, is it in full sun, shade or what, soil condition is a factor too, some plants only require a handful of say, blood / fish / bonemeal once in spring to help give them a LITTLE feed, but over feeding gives either too much foliage and soft to the extent they get attacked with insects etc, or get diseases like grey mold etc, so really you need to check the growing conditions of your vine where you have it growing, maybe your just being over kind to it.
Good luck. WeeNel.
I had a very old one that would look dead well into June (People often commented why I didn't get rid of it if it was dead.) We had to move it when we built a garage and it never took. I bought another one, planted it in the east near the house. Big mistake...it grew under the siding. I've moved it now (although it still comes back in the old place) and it totally covers the wrought iron wide post. It blooms profusely. Unfortunately, it keeps trying to spread. And I don't do a thing to it. Never water it or fertilize it. I do clip it cause it would take over the whole awning if it had the chance and probably strangle me while walking near it. It comes up in the lawn three feet from where it's planted. I'd probably never plant one again.