With infrared systems the audio is transmitted via a beam of infrared light. In order to operate the receiver (headset) must be able to "see" the infrared light being created by the transmitter (base station) hence the receiver must be "line of sight" to the transmitter. If the receiver is not "line of sight" to the transmitter or is too far to one side/up/down and is outside of the infrared coverage area or something blocks the light between the transmitter and receiver the audio signal would be interrupted. If the receiver goes beyond the range of the infrared light (ie: too far away from the transmitter) the infrared light will have dissipated and the receiver will no longer be able to detect the infrared light. For optimal range ensure that the transmitter is placed to provide a clear line of sight to the receiver. Often placing the transmitter is an elevated position with a slight downward angle to the receiver is optimal.
Another factor that can affect range for an infrared system is lighting within the room. Some light (especially concentrated light like a lamp) will cause the infrared light to dissipate more quickly so ensure that the transmitter and receiver is not directly next to a concentrated source of light.