Is your heat source under the ring of ice?
The ice forms at the top where the water level is at the time. The bottom where they drink stays thawed, but up higher at the water level top, sometimes I get a thin ring of ice. I guess the heat doesn't travel as well up the sides of the plastic. I did have frozen areas on a metal one back in 1994, but that was with -22 temps at night. What a wonderful first chicken year that was! I sometimes get some ice in the drinking part below 10 degrees. The heater I bought 2-3 years ago doesn't seem to get as warm as the one I purchased in 1993!