Adult readers - getting feedback

We had some family around this weekend to stay.  There are the two girls - T and M - and their parents - Jenny and Jake.  T read Hacking the Dragon in its pre-edited state several months ago (her verdict, "awesome, Uncle Mike" did great things for my ego!), but Jake hadn't been aware that she had.  We were discussing it over supper last night after the kids had gone to bed, and he expressed an interest in reading it.

I've generally taken a view that the more people who read it, the better, as I'm very keen to get critical feedback, so I sent a Kindle-formatted version to his phone, and he started reading it ("I'm enjoying it so far", after the first chapter).  The problem this raised again for me is that a number of adults have now seen copies of Hacking the Dragon, but I've only got feedback from one of them.  In fact, as far as I'm aware, he's the only one who finished it.  (Thanks, David!)  Why is this?  I can see 3 likely reasons:
1) people aren't getting round to starting it;
2) people start it, don't finish it, presumably because they're not drawn in, or maybe just because they're very busy;
3) people finish it, but don't give me feedback.

Kids, on the other hand, are no problem.  They'll read it - quite quickly, in my experience - and give honest feedback.  And so far, it's all been good.

The problem is a combination of the fact that I want to get feedback from adults (who are generally more experienced readers) and that I don't want to be seen to be hassling people too much.  They've kindly agreed to read something for which they're not the target audience, and I already feel that I'm imposing somewhat on their hospitality.  To be seen to hassling them is either rude or desperate - or possibly both.  And then there's the question of why they don't get back to me.  For either of the latter two reasons, I'd really, really like to know why.  I'm not going to be offended by their not liking the book: no book suits everybody.  It could be that they just don't think it's that good.  Well, I know that it's not perfect, and one of the main reasons I gave it to them was in the hopes of getting some feedback to improve it.  Maybe they think that this is just a hobby, and that's how it started, but now I'm committed to this, and I'd like to make a career out of writing - even if it's not my only means of income.  Feedback is important.

So - frustrating, but I think I'm going to have to bite the bullet and start asking for honest feedback.  Beware, friends and family!