This may be because you have set a "Maximum number of competitors".
You can use the drop-down menu on the right to set the "Maximum # of Competitors" to the maximum number of competitors you allow per event / per category(ies).
Even if you have set it so that you want to manage the reserves manually, Athletics.app will still keep an eye on whether you have already reached the maximum.
Suppose you have set a maximum of 10 competitors for High Jump MJD1 and you try to allow 11 competitors, then only 10 will be admitted and the 11th will be rejected by Athletics.app. As soon as you have changed the "Maximum number of competitors", you can then allow the competitors after all.
The same applies to the maximum number of reserves. Once that number is reached, the next athlete in line is automatically rejected.
This has been automatically translated into English