Most likely, it is one of two things;
1) They are not a confirmed participant or coach.
2) They only created a Member Account and did not actually register into your event.
The most likely of the two is the second...this is a very common occurrence and means that the parent or coach only made their "universal Stack account" in your system, but they didn't complete a registration form to any event of yours.
Sign up for anyone new happens in these two parts;
- Member Account creation
- Registration to an event
When a parent or coach clicks on the Register Now button at your "yourclub.osysalive.com" site, or uses the direct URL from these buttons, it has them do both in progression at one time. The key here is that they have to keep going.
Stopping in the middle of registration will save their progress, but it only creates a Member Account - they must finish the registration in order to show in your backend.
We have also found that most parents get stuck on this page:

They just need to hit Continue Registration on this page to continue forward. They are not done.
So if you can't find a Coach or Player, they need to click the Register Now buttons or use the direct link you sent out, log in with the account they made, and continue through registration until done.