Can I suggest something I've nicknamed 'Beige Rock'?
It's a bit like greyrocking somebody, but it's slightly warmer for the ambiguous possibly sadfishing/possibly anxious/possibly depressed.
'Of course you were missed! I was thinking of either Cheapandcheerfulpizzaandpastaplace or Wankyspanishtapasthatcostsafortunebuthasamazingbogs for next time. As you missed last week, you get first choice on the venue and I'll let everybody know - I'll need to book by Thursday.'
And if she doesn't respond 'I've booked Cheapandcheerfulpizzaandpastaplace for the 15th - I need to confirm the final numbers by the 7th. Looking forward to a proper chat!'.
So you're not ignoring what she said, but you're not letting it become a big, dramatic thing and you're being chirpy and positive without being manipulated - if she's feeling down and insecure, that might be enough to get her there and if not, well, it's up to you if you do an 'Are you OK? You've cancelled both times at the last minute when I know you wanted to come'.
And if she cancels again 'Oh, that's a shame. You're still welcome to anything we do, so don't worry about that, I'll keep on inviting you until you tell me to stop'.
DP informs me that the regular chirpy messages and comments where he wasn't dismissed or told the awful 'I'm sorry you feel that way' (which means to many people 'You're being completely unreasonable and what's more, I don't care, I'm just saying this as a politer alternative to Fuck Off') were what got him to risk actually coming out and interacting with people again when he was depressed and anxious. But had he had the 'Sorry you feel', was told off or cut off, he'd have spiralled even deeper into depression.