The base unit of the RS 120 headphones has an automatic shut-off which puts it in to stand-by when there is no sound going in to it.
If the sound from the TV is too quiet for too long it thinks you have finished using it and switches off. The green LED on the base unit will go out when this happens. If your headphones are connected to the headphone socket of the TV, try increasing the volume of the headphone socket from the TV controls. You may then need to reduce the headphones own volume control to a comfortable listening level.
If your headphones are connected to a line level source you may need to try connecting them to the headphone socket instead.
The red LED indicates charging and should only be lit or flashing when the headphones are on the hook, being re-charged.