There's a upper size limit of new APKs that F-droid will include. If they're too big, they won't be considered...

Sometimes if an app (that's not too big) gets turned down by the F-droid team, contact Izzy via https://apt.izzysoft.de/fdroid/ and he might be able to add it to his repository.

He's super friendly and helpful!

Plus, add his repository to your F-droid and you have Obtainium on F-droid via his repository!