Can't do reverse planks!

I cannot do a reverse plank to save my life, and it is starting to frustrate me immensely! I am fit, I have decent strength, and I always mix up my workouts to get a good well rounded program. My core strength is excellent, and virtually every other exercise I can breeze through. Why can't I do reverse plank!!?? Yesterday I got my husband to try (he is not as fit as I am) and he can do them perfectly - as can my two daughers (13 and 11). He was laughing about it with a couple of friends (neither of them are particularly fit) and they both tried and could do it easily.

I am starting to wonder if it is a flexibility thing rather than a strength thing - I don't have good flexibility in my upper back and shoulders, I can't bring my elbows up behind me very far at all - so maybe I need to do some more stretching instead?

Ok, rant over :) Any suggestions as to stretches/exercises that might help would be appreciated :)