Dagbigboy's explanation is dead on. It took me a long time to figure out how to get monokote to stretch around compound curves. I use a trim iron for compound curves and do a little at a time. As Dagbigboy said, you have to stretch it by pulling fairly hard. I bought a 4" diameter round wooden ball from a craft store and mounted it on a dowel with a base.
I then practiced covering the ball with monokote scraps. I was able to apply monokote smoothly around 1/4 of the ball. If I tried any more than that, it would stretch too thin. I have found that different colors or different batches do stretch more than others, but I have not had a problem after I figured out how to do it.