While the overall transmission range for wireless headphones is mostly determined by the wireless technology being utilized by the individual wireless headphone product there are multiple external factors (like obstacles, audio signal strength, wireless signal congestion, etc) that can effect the wireless signal transmission range of wireless headphones.
Obstacles are one of the main factors that can effect the transmission range of the wireless signal. While the wireless signal (with the exception of infrared signals) can pass through obstacles (like walls) ultimately range will be affected (likely reduced) by passing through these obstacles and the more dense the obstacle the greater the effect will be on the signal. For example a signal passing through a glass window won't be affected very much whereas a signal passing through a concrete wall will be effected much more greatly. Having a clear line of sight does provide the best transmission range.
The audio signal being output by your audio source to the transmitter of the headphones is another key factor in transmission range. A weaker audio signal going into the transmitter means the transmitter has a weaker signal to work with which can in turn reduce the transmission strength which ultimately affects range. Ensure that your audio source is outputting the best possible signal for the transmitter to use.
The wireless signal environment also plays a factor in transmission range. Many devices (like wifi routers, wireless printers, etc) also occupy the wireless signal space where the signal of the wireless headphones operate. The more wireless devices in operation the more congested the wireless signal environment and hence the less flexibility the headphones have in selecting a wireless signal path to use. The further the headphones get from the transmitter the weaker the transmission signal and if a more dominate signal is trying to use the same wireless signal path the more dominate (ie: stronger) signal will overpower the less dominate (ie: weaker) signal. Turning off wireless devices that are not always in use (ie: wireless printers) can help create a less congested wireless environment which allows for wireless signals to spread out and locate individual wireless signal paths rather than trying to use the same wireless signal paths.
Another factor that can effect transmission range is proximity of the transmitter to other electronic devices (like wireless routers, wireless phones, etc) or to large amounts of metal (like metal shelving). It is best to make sure the transmitter is away from objects like these to ensure best wireless signal transmission.