Try removing the antenna from the collar and cleaning the contact points real good.
I agree...these things are a bit fickle sometimes......but Lord they're so much better than the old radio collars. Just so you know Mitch,.....the first two DC40s that I bought enevtually had to have the antennas replaced, Garmin did this under warranty.....I then replaced them with tuff-skin antennas from Conkey's supply and haven't had that happen again.