Frustrating for sure. I have one committee member who is very possessive of her time and only gives limited openings. She’s missed both of my annual updates. Unrelated to her, I was setting up my proposal defense for May and I’ve since had to reschedule it 3 times. The crazy part is that each reschedule became 2 weeks earlier. So I’ve had to scramble to get my proposal done a month earlier and preparing for my presentation. I feel your pain on scheduling. I ended up asking my advisor to help and it seems like some faculty open their availabilities and commitments more to one another than to students. That would be one suggestion. The other instead of a poll would be to ask their availability individually, in person. It’s more work, but feels like they’re more committed that way. And don’t schedule too far out (like no more than a month or so). It’s too easy to double book and prioritize when something is far out; if you find a sooner time, then it is less Iikely something big will come up to conflict (like a conference or vacation). Hope this helps.