Ha ha ha. I was actually having that same problem at the beginning of this outdoor season. It all comes down to how you set up your workouts. Now I don't know what you do, but this is what I WAS doing and how I corrected it. . .
Before I changed it I always had them run at a certain percentage, usually 90-95% (what most coaches do) but what I did wrong was base the percentages off of what their season best time was.
The CORRECT way to do it is to gradually increase intensity, or gradually lower goal times, regardless of what their season best is. This way, you are constantly and consistently taking them out of their "comfort zone", causing them to practice faster, which in turn causes them to compete faster (i.e. – faster times).
I hope this helps.