as stated above.
Any ideas?
Regards,
herz.AG
What K-9 Mail version are you using?
Does checking individual accounts lead to the desired results?
actual version 5.517
checking all accounts manually does work. but with 15 accounts+ you cant do this each time individually. it is so “whaaat?” 
Sorry, I wasn’t trying to suggest that you check all accounts individually as a workaround. I asked because the answer to that question can narrow down what could be causing the behavior you’re seeing.
Since checking individual accounts does work I don’t see why using pull-to-refresh in the Unified Inbox shouldn’t work 
Just to make sure I’m looking in the right place: You’re opening the Unified Inbox, and in the message list you swipe down to refresh?
How do you determine it isn’t working? Are you sending a new message to one of your accounts and that message is not downloaded when you sync the Unified Inbox?