Ahh, I remember well how frustrating the mis-gendering thing was, ugh!
Unfortunately, getting friends to get the pronouns right did take some time, more time for some than others!
My advice is to be patient, but firm, and to try to remember that people knew us one way for a long time, and when they are remembering stuff, in their minds, and sometimes even our own, we see a picture of the person as we believed them/ourselves to be, and our words reflect that reality.
I do know it became much easier when I went full-time and my appearance reflected my true gender, but even then, people still slipped up in conversation. I was passing everywhere, and in one instance in particular, a friend kept telling a waitress that 'he' wanted more tea, lol, and she would look at my friend like she was crazy, and I felt outed and vulnerable, but I learned to just play that it cool.
So, yeah, there are a lot of reasons people might slip up, but we are all human, so be patient, and calm, no matter how many times they slip, they will eventually get it right. If they are really our friends, they will be trying the best they can.
When people we know mis-geneder us for passive-aggressive reasons, well, that is a whole 'nuther topic, ugh!
Hope this helps.
Missy