K-9 doesn't "see" public keys in OpenKeychain

I am trying to get my K-9 gmail address to work with OpenKeychain. In OpenKeychain I have imported my full key and public keys from several others. I can send encrypted mails with no problems (until now), but when I receive mails, I get the message “This key is not available. To use this key, you have to import them as one of your own keys”. I have searched everywhere if someone has ever had the same problem, but I can’t find any solution. I can’t find any possibility to import keys within K-9, but all the necessary keys are in OpenKeychain.
I would appreciate if someone could point me in the right direction.
Thanks in advance

is K9 connected to OpenKeychain and is your key selected under “Configure end-toend key”?