Don,
In my experience, the Cab-1 functions fine until the batteries get down to about 1.2-1.3 volts each (4.8-5.2 total). Your Cab-1 could just be going to sleep on you. Since there is no "on/off" switch on a Cab-1, they are made to turn themselves off after a period of inactivity. Turning the knob will not wake them up, a button push will.
The timing from last command issued to the Cab-1 turning itself off varies. Some of mine will take 4 or 5 minutes, a couple of them only take 1.5 to 2 minutes. If you notice the knob not working, blowing the horn/whistle or just pressing the AUX1 button will make the knob work again.