What sort of training program did you use? If "hours on the trainer" was the majority of it, you might have an explanation for your comparative lack of improvement right there.
If you haven't already, get out there and do some interval training. Heavy duty interval training. The type that will make you feel like, if not really, puke at the end. Up the distance or time of each set of intervals every couple of weeks.
Do you have a heart rate monitor? If not get one; if so... link it to your training program.
Endurance is one thing, and that comes from the trainer and long steady distance riding. But when it comes to improving speed, you have to step up to high-intensity intervals, and practise riding over courses of similar distances.