We have all different types of hebes; and I'm not sure why, but they don't seem to be thriving. They do get watered at least twice a week and are in ok soil. We have fertilized them. There are two that seem to be ok, but two others have died and the other five look really 'sad'. They get full sun in the morning and afternoon. They seem to have brown leaves in the bottom middle of the plant that easily come off (look like they are dead). Any advice?