I have a friend who I'm very fond of and who I've known for almost a decade. She's always been a little bit mysterious in that she will appear and reappear after unexplained periods of silence, decline/cancel invitations without providing a reason or even disappear at events without warning. For example, at my engagement she offered to pick up my friend and go with her, but while there she left abruptly without telling my friend, leaving her without a way back home 
For the reasons above many of our friends have essentially given up on her and stopped making an effort. She still gets invited to group events but there's little expectation she'll attend or even respond. But I've really persevered because she's a wonderful person and also because she has opened up to me over the years more so than the others e.g about a past abusive relationship she was in. She's also occasionally dropped hints about things going on in her life currently which leads me to suspect she is dealing with something quite terrible.
I've tried to be there for her in the limited way I can, which isn't easy as she refuses to tell me exactly what it is she's going through. But like our other friends I have been getting more and more frustrated at her vagueness, which came to a head last week when I invited a bunch of friends over for a farewell party before I left for overseas for an extended trip abroad. I especially went to the effort of messaging her privately rather than just sending a group message and she asked what time it was and other details before going silent on me when I asked if she was coming. She didn't let me know either way and then didn't show up, despite knowing we wouldn't see each other for a long time. When I messaged her afterwards asking why she didn't at least respond, she replied that she 'has a lot going on' and that it's 'very private and has been going on for years'. I reassured her, as I always do when she hints at things, that I was there if she ever wanted to talk, to which there was no response.
I feel like I'm reaching the end of the road here with her - I want to be a good friend but I don't see what else I can do and I'm also annoyed at her constant flakiness. AIBU to leave the ball in her court now? I feel bad because she really does seem to be dealing with some issues but it's difficult to keep making excuses when I have no idea what they are!