Rear windows leaking, 03 plate Fabia...

I was washing my car today with the jet wash and noticed a large amount of water running down the inside of the window, and when I opened the door to check the window was shut properly the seats were soaked too. I tightened the window, tried again and it was still leaking. Same with the other side, but not as bad. The front windows were fine. Is this a common problem? I haven't noticed it during rain or anything...