What does she mean she will 'try' to be there? A - she needs to let you know if she is coming, bridesmaid or not. B - if she is your bridesmaid she needs to be there! No 'try' about it.
Her being the other end of the country shouldn't make any difference. One of my bridesmaids lived in Florida when I asked her (I'm UK). She was up front that she might not be able to come to the hen do, which is fine, but there was no question about her coming to the wedding itself! As it is, she's now moved to Vienna, so is now coming to the hen do too!
That said, as long as she tells you she is definitely coming, I don't see what the problem is - has she already agreed to help you with prep in the run up? If so, you'll have to rule that out, as it obviously won't be practical. But if you're just expecting her on the day, I don't see what the problem is. I probably wouldn't have invited her as just a guest instead - if she can't make the effort to be there as bridesmaid, being demoted isn't going to make her more likely to attend.
I would have a straight chat with her, tell her you know moving is stressful, but you need to know for sure if she is coming - if only for numbers and catering!
Good luck X