I've had a lot of interactions like that, but face to face. Blokes I went to school with and not seen for 25 years, bumped into them in another town or soemthing, went up and said "Hi Jon, remember me?" and they reply "oh yeah" and walk off! And these were guys that I was fairly good friends with.
Don't take it personally, I don't know if it's a memory thing or a personality thing, but I remember everything from school (names, faces, what we used to play at playtime) yet some people I meet remember virtually nothing. All you can do is reach out, and if it's not taken up then leave it. Also remember that some people who were nice 30+ years ago are not nice now. Sad but true