I don't know but I have a similiar problem I think the last time I had more than one or two flowers was the summer of 2006. Where I live we had alot of rain that year Last year and this year so far have been drier than usual. I have three one is a hybrid and never bloomed They are six and seven years old.

I just noticed one bloom and a japanese beetle I think they are eatng it too as well as other insects

I don't think it's japanese beetles, (yuk I hate those !!) I think it may be too much moisture or humidity in the air, not enough circulation of air thru the branches. Prune it back in the Fall... a good cut back; also prune out some branches here and there in Spring once it starts to fill in ( you might lose a few flowers ) to let some air/sun to flow thru a bit. I had this happen once, and I believe it was because we had toooo much rain and humidity... and trumpet vines can get so dense. I prune mine back in the Fall severely, and try ( try ! ) to keep this rampant grower under control. I haven't had a problem since, and we have lots of humidity in the summer.

I've heard that too much nitrogen fertilizer makes them focus on vegetative growth instead of blooming.