AIBU expecting grandparents on school drop off

[Kate1556]

Recently I was asked to go away for work, 3 weeks before I left I asked my DM if she would be able to drop DD off at school, she said it wasn’t a problem.
I found out 2 days before that she hadn’t asked to change her hours at work and couldn’t do it.
I asked why she agreed to start with and she had a huge list of excuses.
She thought DD was in before school care (she never has been)
I asked too late
When I reminded her I asked weeks ago she said I asked too early
she does this for my DN all the time, amibu to have hurt feelings that she didn’t tell me, never apologised and has been able to do the same thing when my brother is in the same position?

Different from the question in your title, but YANBU to expect anyone who has agreed to do something for you, to actually then do it - or let you know as early as possible, with profuse apologies if for some reason they can't.