No fertilizer on things like grass, that may be growing close to the vine? I've never had a problem with mine.
Do you prune it, to keep it under control? Trumpet vines bloom on new growth, so if you prune it in the spring, you might be cutting off the buds.
The only other thing I can suggest is that, according to what I've read, they have to be around 4 to 7 years old before they bloom. Is yours, perhaps, a young plant?
That about exhausts what I can offer regarding trumpet vine ... except to keep it far away from your house. It got under the shingles on one side of my house, and I had to have them all removed in order to get the plant out. The roots actually grow into the wood. It was a very expensive lesson for me.
I love my trumpet vines, but I strictly confine them to the chain-link fence now.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams