K9-Mail does not detect PGP encrypted mail automatically

I have a bit of a problem with PGP and K9-Mail on my Galaxy S3. I'm using the GPG Mail plugin on my computer (Mac OS X 10.8) and can read and write mails just perfectly. Today I decided to setup PGP on my phone. It's rooted and runs the ParandoidAndroid 3.55 custom mod. I did the setup as shown in this guide: http://cariadus.com/blog/2010/07/how-to-set-up-your-android-phone-to-enable-sending-pgp-signed-emails-to-nominets-automaton/ and everything went fine. I can encrypt and decrypt mails sent from my phone (to myself). BUT, here is the problem. As soon as I send myself (or got a mail from a friend) that wasn't sent via K9-Mail and Android it doesn't recognize the signature. FOllowing screenshot will explain: It shows me the attachments but the usual "Decrypt" button that I get from mails sent from my phone itself doesn't show up. I update my public key + the public key from the friend that sent me a mail from the keyserver and have both of them stored in APG. Well, I hope you understand my problem now and have advice on what I can do to fix this :P Thanks!