Hello Rob123, welcome to PCHF.
Not sure what cable your using there, Its sounds like your most likely using a mono lead instead of a stereo one. Some systems will recognise that you have a mono lead attached, others won't and will output to the right channel as you've described.
Have a look at your connections, a mono lead will have a single black line towards the front of the jack, whilst a stereo will have two.
Mono Example:
