I started out a VA myself. In my seven years in Upwork working as a VA, I rarely had problems with clients. One client, in fact, I have been working with from day 1 until now! When I do have to end a contract with the client, the one reason that always pops up is a mismatch of the job description and the actual work the client gives. For instance, one client said he needs a VA. When we started the job, he asked me to code email templates and do graphic designs--none of which were included in the job description.
Maybe you have to be extra specific with what you're looking for? I am currently happily working with a client as her project manager and she has referred many other clients to me so I can train VAs for them. We've been operating for half a year already and have been a success!
Hope this helps
I'm sorry to hear you're having trouble finding a suitable VA. The problems could be any of the above that others have mentioned. One solution, however, is to have a procedures manual that lays out the steps in the different tasks you want your VA to do so that there is no question about what's expected. It's very time consuming to create them but can save a whole heap of misunderstandings once they're done. It can then be the VA's task to keep them updated if processes change.
The other thing that makes a procedures manual a great idea is that it can be passed on to the next VA if someone you've been working with is unable to continue with their contract.
I've recruited staff on behalf of clients (while working as a VA) in the past and I would never make advertisements public. I'd contact people individually and invite them to interview which helped me select only those who I thought were going to be a great fit. I'd then proceed the way you did - creating a list of interview questions, speaking to them on the phone to ensure they understood what was involved and setting small tasks at first to see how we both worked together.
It's been my experience that when working virtually one has to communicate a hell of a lot more and a lot more precisely than one normally would in a physical office - that's where the procedures manual comes in as it saves having to repeat one's self over and over.
Hope some of this helps!