If you hadn't said you used this same headset/mike on another rig, I would have thought that perhaps you weren't speaking closely enough to the noise cancelling mic element, which reduces drive very quickly.
So that has me leaning towards a defective cable, or perhaps a defective mic element.
I agree with Ria, Call RadioSport Tech Support....
Problem Solved by Dave at Radiosport, thank you!
I feel such an idiot ....! Whilst I had enabled the Bias in the "Settings/Phone CW Tab", I had not clicked on the separate 20db boost button. When I did this there was plenty of audio gain!
For info I show below Dave's advice:-
"We have recently dropped the -ADJ nomenclature for the M350
Mic (part numbers getting way too long when appended to headsets) and yes all
M350 do have adjustable gain, although virtually everyone runs them at full
gain. Be certain that the +20dB Boost (next to the BIAS) is enabled
(highlighted Blue just like the BIAS) as I've seen this disabled if the Mic
Profile has been saved without it enabled and so each time the radio is started
it goes back to the profile setting. If you have verified this then there
is something wrong.
Please note we have only had one M350 failure in 7 years and that Mic was blown from a static discharge (handling the headset in very dry conditions without discharging one's self before picking up the headset). That type of failure is complete, the Mic won't work at all. If yours works but has low output, please have xxxxxxxxxr at xxxx check it out in their workshop. They will be happy to replace if it is defective."