Thanks for the replies, everyone. My goal was to have a constant beta reading project on the go since the new year, and in doing it repetitiously, I've discovered there is quite the learning curve.
I try to communicate as efficiently as possible, establish expectations first for style and deadlines beforehand - which also means I have email correspondence with everyone before starting the project.
I now ask for first chapters. I started off very indiscriminately, but realized it's better to make sure I'm the right person suited to the manuscript (and vice versa).
It has made me question if it's something *I'm* doing poorly. I try my darnedest to make everything helpful, supportive, and constructive. Not, "this page sucks," but, "I like XYZ - can you explain more about it here?"
Like frimble suggested, trying to think the best of people, you just don't know what else a person has going on in their life. Especially if writing doesn't pay the bills, sometimes things come up, and writing tasks just have to be set aside.
This is why I stepped back and took a few deep breaths. I know I don't reply to emails every day. (But I would at the very least thank/acknowledge someone for their time.)
My gut is telling me crits can be a lot to digest. The common denominator is that I've critted mostly first-time manuscripts.
the opinion that critiquing/beta-reading was a gift more to the beta reader than the author. That's not my view, but it helped me to know that view was out there.
Interesting perspectives. Personally, if I posted a thread publicly for critique, I would likely give public thanks + rep. I would be less offended if someone ignored a short public critique of mine, than a huge private project that they sought out and requested from me personally.
Whether or not someone says thank you, I still had the opportunity for the exercise in critical thinking, so IMHO the benefits of critting are a two-way street. I've also connected with some insanely cool, inspiring people through beta reading.
Anyway, I don't want to sound like a gigantic grumblepuss, but I was sincerely curious about other experiences, and whether or not you just move on with life if the author is Houdini.