A little grumbling on my part...
Slightly more personal grump, this one related to my writing projects - I realise that people have other things going on, but I must admit it does get a little irritating when someone says to me that they will proofread my work or give me some feedback or something like that and then when I give them a verbal nudge asking for their feedback/proofread response, they simply say they never got around to it.
To be honest, it has gotten to the point where I am reluctant to ask, because if I had tried to do it for something like my Creative Writing Project manuscript for university, I would have been royally shafted, because that had project milestones that I would have missed because of other people not keeping their promises.
Sadly, with writing books, feedback is needed, otherwise people simply will not pick up your work - making it a bit of a destructive process if you can't get an initial set of comments to get the ball rolling.
Like I said, I understand that people can be busy, other things can come up, but I wish they would tell me if they were unable to do it for whatever reason so I know to look for someone else, it's not really fair on me if I go back to them 3 weeks in a row, each time they say "I'll look at it this week and get back to you" and I'm forced to simply say "Don't worry about it, I'll find someone else", having wasted nearly a full month of time that I could have used editing my work.
At the end of the day, please remember that if you say you will help with something like that, you are making a commitment - if you can't do it because something else comes up, let them know, don't just leave them hanging and waste their time.