Fast track!!

Hello, After sending my daughter passport 30/1 and still not have it back we are thinking of using fast track as we fly 4 weeks on Thursday. Can someone explain to me how it works if you have an application already going? Hers is a replacement for a lost passport. So we can get another full birth cert if needed. Current application status 30/1 application made 18/2 documents received 21/2 processing 12/3-16/3 got a letter saying they needed additional documents. 21/3 documents delivered and signed for. No update since. We tried ringing and we are waiting for a call back as apparently her Application isn’t on her system.