GARMIN SPEED SENSOR 2 not listef

I’m a newcomer to Zwift. I have a fairly old Tacx trainer which doesn’t link with Zwift, but I have fitted Garmin speed and cadence sensors to my bike. The cadence sensor is working fine but the speed sensor is showing on the pairing screen as ‘not listed’ even though it appears to be paired with Zwift. Anybody else having this problem or know how to resolve it? Thanks.