It could also be subtle things in how you present yourself -- body language, facial expression, tone of voice, word choice when answering interview questions. Do you have anybody who could do mock interviews with you, help you to identify and improve any issues with your nonverbal presentation or how you answer questions?
This is definitely something to look into.
I dont work anymore, but back when I did, I was *terrible* at most jobs... yet I'd still get more of them as I'd learned just how important the interview itself is, and how to effectively "win" in it. Which might sound like a strange term to use, but that's one possible way to look at it... you're effectively competing with all the other applicants, so you need to do something that puts you ahead of them. And a cover letter or application is *not* enough. Those things just get you in the door. It's what you do once you're inside that's going to be the real deciding factor. Or what you DONT do.
If you're having this much trouble getting employment even despite all that volunteer work, there might be some aspect you're not aware of in the interviews that's essentially shutting you down. Something you shoiuld be doing in each one that you arent, or something you are doing that interviewers as a whole really dont like. And yes, it could be something as simple and ridiculous as some small aspect of your appearance. Or even your voice, how loud/quiet you might be. Even silly little things like that can win/lose you a job once you're in the room. The problem with interviewers: They want perfection. Nobody is perfect, but they want it anyway, so they WILL pick apart every aspect of you that they are able to witness to spot as many flaws as they can.
Of course, actually figuring out what the issue is, is the whole problem.
The one thing NOT to do though: Dont think of this and just go "But I'm doing everything just fine!". That kind of approach (the refusal to see problems that may exist, LOTS of people do exactly that when faced with frustration like this that involves some task they need to complete) will just make this that much worse. If there are problems, you must figure it out, even if the problem ends up being something bloody stupid, or something you dont like dealing with.
The trouble of course is actually figuring out, well, how to figure it out. Is there any way you can speak with a professional about it? The sort of people that are there to help get you jobs, might be able to do the "mock interview" that others had mentioned above. Which could help greatly. Of course, this is all assuming that they dont do things like say "you're doing great!" just to please you even if there are big problems... that doesnt help anyone.
Good luck either way.