Yes. I've experienced this. I reached out to those who I wanted to try to keep in my life and was very honest with my sadness that we'd not had contact for a while. I updated with where I was at, health wise, let them know I valued their friendship and would love to catch up, and left it at that. I was very heartened that everyone I contacted responded immediately and resumed contact. I initiated this contact because of my experience as mentioned below- sometimes people need an opening to communicate because they just don't know what to do for you. It's not terribly fair, but it's sometimes the case.
To my shame, I've also been that person who drifted away for a time from another friend who was in need. I'm not proud of it- as a matter of fact, my shame was what caused a lack of communication to begin with. A very old friend was experiencing a crisis I had no idea how to "fix". It wasn't my job to fix it or provide advice, but that's how I saw myself. When I didn't know what to do, I felt awkward. When I felt awkward, I procrastinated and didn't call my friend for a few days after our first conversation. When those few days passed, I felt terrible for not calling. The guilt made me question if I should call because I felt my friend was probably upset with me (she was't, but she would have been right to be) and I procrastinated again. I knew I was in the wrong. More days passed, until I felt so guilty I sent her an email apologizing for not being there when she needed my friendship. We had a very good conversation about what she needed from me and carried on as good friends still- though I carry the guilt for letting her down with me still.
Not everyone will be there for you. Not because of you, or your quality of friendship, but because of THEM. They aren't there because they are ill equipped to deal with real hardship in others- either because of selfishness, or, most likely, an inability to know what to do.
I'm very sorry you are affected by this- it's not fair and must be very hurtful.
Fiona