Hi Mark

Interference mainly

All wireless systems have to use designated frequency’s (Usually set by Government Agency’s) and unfortunately the ones allocated for wireless sound transmission are almost (And in some countries are) the same as mobile phones, thus with the explosion in mobile phone use it has become difficult for manufactures to design a reliable system.

Things are slowly changing as more frequency’s get freed up, so hopefully you won’t have too much longer to wait.

Hope this helps

Bill