The 'Blue Prince' and 'Blue Princess hollies are indeed wonderful plants for year round interest. Leaf drop can be caused by several factors. Check to see that the bushes are not being attacked by scale insects. These are small sucking insects, visible with the naked eye, that can cause defoliation. Also make sure that the soil is well-drained, on the acid side and that the plants have a part shade situation. Since you have already lost some plants, you would be wise to contact you local Cooperative Extension office (yellow or blue pages) and ask for their definitive diagnosis.