Are the roots shrivelly and wrinkly before they stop growing, or are they still fairly plump? Wrinkles tend to mean the roots are too dry. I've seen dry roots that had dark tips spring back to life and grow again once conditions became more humid. To increase humidity, set up a small cool mist humidifier near your orchids, or group them together with other plants.
Are the roots inside the medium pretty healthy? I've struggled with stunted root growth on plants that have raging fungal infections. I had to get the infection under control before any roots would grow beyond "stump" level.
I help my mounted phals out a bit by putting some Spanish moss around the roots, especially when I have to bring the plants indoors. The moss retains just a bit of humidity, while allowing the roots to dry out quickly and adapt to an aerial habit.
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