This is probably because the scoring formula for combined-events is set differently than the scoring formula for 'single events'.
You can set 2 scoring formulas per category, 1 for the combined-events, 1 for the single events.
For example, if you have selected the scoring formula 'KNAU' for the combined-event and there is no scoring formula known for that event, Athletics.app hides the option to use that event in the combined-events. This is because if this were allowed then everyone would get 0 points on that event.
You can fix this by changing the scoring formula for the combined-events of that category. This can be done via the wizard step 3b.
There, select a scoring formula that you know does exist for the 30 meters.
If you can't find a scoring formula, choose "Points per grade. Then each event will become available to add.
Read more about 'Points per rank' here:
https://www.athletics.app/faq/?qa=6/wat-doet-de-puntentelling-punten-per-rang
This has been automatically translated into English