First, make sure it's in the right site for the plant; correct sun, etc. It's possible it isn't doing well because it isn't getting enough light or some other environmental factor. It also may be that it just isn't well established in its new place yet. If you're going to cut it back hard, late winter or early spring is a good time. I've included a couple of links that probably tell you more than you want to know but there's some good, basic pruning information. Good luck!

http://www.ext.vt.edu/pubs/nursery/430-459/430-459.html

http://www.mortonarb.org/plantinfo/plantclinic/hortcare_pruningshrubs.pdf