I think this can be a common problem. My 96 girls team suffered from this. In 3rd Division, they were often the better team and knew it. So they'd play down to the level of an opponent for the first half. Then in the 2nd they'd score a bunch and be done with it. Drove us crazy. So we moved them up to 2nd Division. That freaked them out - none believed they were ready - so they played scared. But still, this past season, every single match we played except for two, the score was 0-1 at halftime. Was bizarre. Then the wheels would usually come off.
That said, I've had to deal with the same exact thing with a couple of my strongest players. Probably my most dangerous scoring threat often starts flat. So we started her on defense for the 1st Half. She had no choice but to play hard and by the time we moved her up front in the 2nd half, she was chomping at the bit.
So maybe explain to the 'starters' that they aren't going to start until they start to show intensity from the beginning. May work, may not. Never did for my team, but may for yours.
Girls soccer teams = amazingly complex puzzles :)
I wish I had better advice, but I'm still trying to figure it out - haven't yet.