I had the very same experience in the last week. Paid for passport express, 10 days later my passport status was still at registered. I emailed and they said it was waiting to be checked. The next day it said it was posted but the one after that, it was back to being checked and the date of receipt of my application had changed from 20th July to 5th August. Once again I emailed as I was very confused about what was happening at this stage and was given some fob off answer about rescanning photographs but nothing about how it went from posted to being checked. As it happened, it arrived the following morning. Cant help but think it would still be waiting if I hadnt chased them on it.
My partner posted their application after me and lo and behold, its due and is still showing as registered only. No response to emails on that one!
I fully appreciate that this is a busy time of year but it annoys me no end to see them have 10 working day guaranteed turnaround on their website etc when this isnt going to happen. If there are delays/backlogs at least post something about them on the website.