I grow these and am happy to help. Just so your expectations are realistic, I want to warn you that they take ages to grow and few varieties will reach the 4 to 6-feet that you're probably looking for. However, you can shape any non-trailing rosemary plant into a traditional Christmas tree shape, but it will be more of a mini tree. Rosemary plants are tender perennials if you don't live in the correct growing zone, so check that out. Also, where are your plants located (in your house, in your greenhouse, in your yard, etc)? My initial thought was that it was root rot, but since you already ruled that out my next guess is that they got too cold or the humidity is off.