Padraic gave you a good answer, and he touched on this, but there are many times, especially when fishing to educated trout, that they refuse your fly at the last second and it's very hard to tell. With all the years I've been fly fishing it still amazes me how they can do this. Just last week they were driving me nuts. It looked like there was no way they were refusing the fly - they would rush up, look just like they grabbed it, I'd set the hook, and nothing. And a couple times they "treed" me too. It seems like there's no way they could change their minds this fast, but apparently they can. I know they were refusing at the last second because after this happens a couple times I don't set the hook right away, I wait for them to grab it. (And when I'm setting the hook, I'm doing it slowly, pretty much the way Padraic described.) Then if I'm still missing them I wait even longer, eventually getting to the point where I don't set it at all, and it becomes obvious that they aren't taking the fly, just splashing at it. The best I can tell you if this is happening, is to do the same thing. Hesitate before setting the hook, set it slowly, and if this still doesn't work, just wait to see if they are taking it at all. Wait for them to grab it and swim down with it. If they are refusing at the last second, you need to try to improve your presentation and/or try other flys.
Oh, and for little chubs, if they are real small, it's possible that the fly was too big for them to get in their mouths.