I had the hardest time trying to catch real punches to do waza, because they almost always retract. However two things that Doran sensei said at a recent seminar were extremely helpful to me. (paraphrased and massively interpreted of course)
1. All the blend you need for tsuki is to try to see who uke is trying to punch. They are obviously behind you so look at them..
2. If you imagine a tsuki as a train, you don't want to catch that train when it's left the station and is full speed, catch it in the station when it gets back and is much slower.
(short rant about atemi's that has no place here..)
I hate it when you atemi someone and they just stand there. You shove your fist right into their nose and no reaction what-so-ever. DO PEOPLE NOT KNOW HOW TO PROTECT THEMSELVES ANYMORE!?!?<shift 1>!!