I'm a bit confused - you organised a hen do last month - did your MOH go to that, or was she invited? If she went to that, I can see why she thought the weekend was free and booked something in. If she wasn't invited, frankly if it was be, I'd be annoyed at not at least having the option to go! I'm a bridesmaid this year, and I'd be upset if the bride arranged a hen do for herself and didn't invite me - I'd question why she'd asked me to be her bridesmaid! Did you make it clear that you were still planning something on the original weekend as well, and give her the option of attending either?
If she's booked something and it can't be changed/rearranged, then go ahead and have your second hen do without her, but 2 weekends away is a lot of time (and money!) for people to commit to one persons hen do, even if they are a bridesmaid.
I've got 6 BM's for my wedding next year, but 1 lives in the USA, 2 have just had babies, and 1 is in the middle of her own wedding plans, so I've delegated my hen organising to the 2 with less commitments, and the others will help out with what they can. Why weren't your other bridesmaid(s) helping her out? Some people are just naturally bad at organising!
Anyway, as the poster above said, I'd have a chat with her and make sure everything is ok with her, and if she can't make it, she can't make it. If she is otherwise a good friend - which I'm assuming she is if she is MOH - it's not worth falling out over.
Hope that helps!