is there a trick to doing boxing punches correctly?

[bfit]

I have been doing kickboxing workouts for years, and I enjoy them, but I still can't seem to get my punches right. My kicks look fine, but there is just something off about my punches (any of them). I look like I'm flailing around in sort of an uncoordinated fashion and without any power. If I do punches very slowly (like in the upper body portion of Kickbox Bootcamp where Keli Roberts uses weights) I'm OK, but as soon as there is any speed involved (like in Powerstrike) I'm hopeless. My husband says it looks like I'm not extending my arm enough, but I don't want to extend my elbow all the way straight, do I? I have followed the tutorial in Kickboxing for Dummies a couple of times now but still nothing has clicked. By the way, I have fairly good upper body strength, and my arms are not getting tired so that's not the problem I don't think. Also, I feel fairly coordinated on other workouts and can follow Cathe choreography and so on. Any suggestions?