Apple Series 1 HR connectivity to Zwift

New to Zwift. Paired my Tacx Smart Trainer to Zwift, also pairs Apple Watch Series 1 & shows it pairs, but HR does not show on watch or downloads when complete. Checked the watch, other HR apps read the Bpm.
Weirdly the first ride measured & reflects the HR on strava measured with iWatch.
So I’m completely baffled!
Please help.