First things I would like at are your water parms, then pests, temp, and lack of nutrients. Zenia need higher nutrients in the water column to grow, we used them to help keep nutrient levels down in our sps setups. Water quality is also important for example Ph and alk levels. then look to see if there are any pests eating them. higher water temps can also cause them to die back and also big swings in temp from day to night. Ph also plays an important role with them we did some tests years ago with the pulsing type at lower Ph levels they would stop pulsing. when the Ph was raised they would start pulsing again. These are just a few items to start with.
Joe