Back in the mid 80s I lived in Norfolk, Virginia and listened to a Top 40 Radio station there. As expected they played only the Top 40 songs in the charts. However, for a while I thought that I must have developed the ability to predict the future as I could easily identify the next song they would play every time. Turns out they would lay out a playlist and not shuffle or mix it up at all, so it was like listening to the same cassette of songs. Once I figured this out, I started looking for other stations to listen to, as I knew what they were going to play before they even played it.
Not saying that is what 107.5 is doing, however, the reason their ratings are dropping may be something as simple as that. Maybe they are getting stale and those teenage girls are looking for something fresh.