I think trying to find the reason why she's not turning up will only drive you mad with frustration. She'll have her reason that will be unreasonable to you and very reasonable to her. My advice would be to be as practical as possible for the good of the unit. Not sure which section you are, I'm Rainbows, so the following may not be good for Guides! It sounds like she's come to you through Join Us, so I'm guessing she's an adult with limited experience of GGUK.
I'd try and meet up with her outside of a meeting, either at her house or somewhere neutral. You can then get to know her a bit, and go through in more detail what happens at a meeting, what she can expect, what the girls are like and, critically, what is expected of her. The first time I walked in to a Rainbow meeting I was terrified of them - I don't have kids and very little family. Suddenly there were hundreds of little people all around me! Up to now, all she's done is fill in a very simple web form, with little to no personal contact. She could think that because your unit is up and running, and was before she volunteered, that she isn't as critical as she actually is.
I'd also set up a parent rota; if she does turn up you can either thank them but say you don't need them any more, or you just have an extra pair of hands, which is never a bad thing.
How bad are your ratios? It sounds like your DC isn't that fussed if this is just "par for the course". My DC is obsessed, but I'll happily run the unit with 1:6 instead of 1:5 - I just don't tell her. It satisfies GGUK policy, just not the way she likes to do things (apologies to any DCs out there - for everything else I do as I am told, promise!).
In terms of how many chances, I think only you can know that. I removed a Guide Helper from my unit's Go! when she hadn't turned up for a whole term with no communication. I later found out that she had stopped going to Guides months earlier, but no-one had told me!
Good luck, hope it goes well for you and your newbie! If it doesn't though, it's not your fault, how can it be if she has nev turned up!!