TOPIC: Participants not showing

We just closed a registration form with over 2100 participants.
Now people are showing up at the event to pick up the pre-registered "bags" for the event. Many are saying they signed up and they are not on the list. Upon checking, they did sign up and were given an Invoice Number by DT, but not on the Records or on the csv export. Anybody experience something like this? What to do? How do you find the missing participants?
Going to lose a big customer over this one, need to find a new registration platform.