Problems with some of my Vandas

[treefingers]

Hi,
I have three different types of plants that are exhibiting similar patterns. The three plants are Ren Nancy Chandler"Red Dragon", Trevor Rath. "Bonjong and a Princess Mikasa. I have had them for several months and take good care of them. Only two weeks ago did they start loosing leaves from the bottom. The stalk on the Ren. s yellow with several leaves yellow to the midway point. I keep them under lights, soak them weekly mist them daily, and there doesnt appear to be any sort of infestation. Can someone help my plants losing leaves, adn yellwing.