Lack of trainer resistance

Hi there,
Since using Zwift with Apple TV, when riding on flat courses, the trainer is not realistic to the course. Eg not enough resistance on the flat. I’m using E-lite turbo Muin B+ with Bluetooth and my Samsung Android as my Zwift companion. I am using my Garmin heart rate monitor via Bluetooth. For example, if I do Alpe de Zwift the resistance and trainer work fine. So the problem is only on flat or group rides. This previously worked fine on my old laptop, Windows and ant+ but not now I have my new Apple TV. Can you help with this as I’m finding it very frustrating. Many thanks