Answer: Well, I don't have the magic pill but I can give you some "points to ponder". Trumpet vine takes a very long time to flower in most every case. I have personal experience and I know exactly how you feel. I have several, and some flowered after 3-4 years, one I think is 7 years old and hasn't flowered.
Trumpet vine flowers occur on new growth (as opposed to flowering on old wood). I would suspect that the plants are busy building up reserves in their root systems to enable flowering. Also, you could be on the edge of their hardiness range and they may not have a long enough growing season to put out flowers. How soon does it freeze and die back to the ground each year? Most flowering plants need a given amount of "warm hours" in ratio to "cold hours" annually to perform as they are supposed to.
Really all I can suggest is that you pamper them through the winter, especially before you get your big snows, and they should flower as the plant ages and matures. As you have already heard, provide lots of organic material such as compost, fertilize regularly with a 5-10-10, and, I would also check the soil. Most vines like a well drained, fertile soil. You could have a soil deficiency that is causing the whole problem.
I hope this information provides some insight and encourages you to be a little more patient with your trumpet vine.
Best wishes with your garden!
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