I have the same headset (bought in 1996) and I flew with someone last week also with the same model although his was only bought in the past 8 months or so. It wouldn't communicate until I used an HMU (headset matching unit) on mine. They must have changed the spec.
I rarely get mine to work with any other headset unless I use the HMU though I too expected it to work with the same model. They must have changed the spec of the microphone.
The HMU costs about £50 + vat
Az