Zwift cadence not correct

Hi there. I run a Elite crono fluid trainer and then wahoo sensors.
Zwift cadence is always wrong and is making group rides impossible. Can anyone help?
When I am doing 80rpm, zwift reads 65 on the flat, uphill sometimes 20!! It never matches the wahoo app running at the same time. The wahoo is spot on. Sensors are new with new batteries etc, all connect nicely. Just Zwift mis-reads the cadence hugely!