https://github.com/seancolyer/gmail-crypt/issues/62
Im having this exact problem.. He never got a solution. I can ecrypt and send.. I even gave my key to a frind to test.. And I coyld decrypt fine... But I'm getting messages from multiple people and cannot decrypt. When I go in APG on my phone to decrypt usually itll say my user name and then I need to type my paraphrase under that... But on the ones I cannot decrypt it says "symmetrical encryption"... Then says invalid encryption data at the top of the page.
I honestly don't know, but try using the program Kleopatra for your encryptions. It's pretty fantastic, and not nearly as hard as what you're doing.