...and a good training software package.
It sounds like most of your improvement so far occured in the first two weeks of your build-phase work, so I'm assuming you were coming off of a month or two of low intensity foundation work. I'm guessing that was more of a muscle tone and conditioning response than the result of your work-outs, there just wasn't enough time for real development.
How would you characterize the efforts in your work-outs and team rides? Are they along the lines of high intensity endurance and lactic threshold work, with hill attacks that exceed your anaerobic threshold for several minutes at a time? If so, than maybe you're already getting your intervals?
Could be, too, that you just aren't recovering quite enough to reap/see the benefits of your work. If you haven't taken an easy week since mid-February, you're overdue anyway. Take a week of much-reduced mileage and intensity, and then start back about 10% easier than before the rest week and continue building for a month. I'll bet your numbers for the week after the rest week will be a happy surprise.
Finally, sometimes just looking too closely at numbers can be counter-productive. Improvement takes time, and if you follow the basic principals training works. Trust your training. If you have a copy of Joe Friel's Cyclist's Training Bible, re-scan Chapter 2, and the "10 Commandments of Training," a quick and easy way to calibrate what you're doing against the basics.
Good luck for a kick-@ss season!