They may be a variety that is long and lean and nothing you can do will make them a truly full tree. Don't cut too much off the top or you will have only ugly shoots. Perhaps in the springtime a good feeding will help them leaf out more fully. Different varieties are like people; some long and lean, some round and full.

Personally, I just love some of the one to three trunked specimens. Given time, the trunks will thicken and look quite nice.

On the other hand, I also like the ones that have a few more trunks or have just been allowed to grow into a bush with no pruning :)

Just as long as they haven't been wacked on top. Just ruins the grace of a crape. Never take off a branch bigger than the diameter of a pencil unless you take it off all the way to a trunk like a tree (and also rub off suckers that you don't want) in that area until it stops suckering.

If you want to play around with one of your crapes (as an experiment), it might be worthwhile to stimulate one to sucker at the base. This can either be done by shoving a shovel or spade down into the roots in a couple of places less than a foot from the tree (usually 6-12 inches from the base depending on size of tree). I've 'fixed' more than a couple of badly chopped box store bargains by doing this after the trees were planted and established. Then, when you have some suckers coming up, selectively keep the ones rubbed or pruned off with your finger that you don't want.

I also took the plunge and wacked one particularly knobby one that was about 12 feet tall down to the ground early spring. I had my reservations about doing this, but it's already 5 1/2 feet tall. Again, you'll have to stay up on the suckers and continually remove the ones you don't want if you try this.

I love the bushy ones! The trunks are nice, but the bushy ones put on a better show. I had three that got snapped off in a storm, and I just let them grow back in bushy. I love them! Flowers galore and pretty green leaves. They do look sort of weird in the winter, though.