Troubleshooting HR sensor mode
In heart rate sensor mode you can turn your Polar watch into a heart rate sensor and share your heart rate with other Bluetooth devices like training apps, gym equipment or cycling computers.
If your Polar watch shares your HR with your mobile phone instead of the gym device
If your watch shares your HR with the paired mobile phone (for example iPhone) first tap With iPhone, then tap 
to stop sharing with iPhone, and finally choose Add a new device again. Another option is to temporarily turn off the mobile phone's Bluetooth connection before you try pairing your watch with gym equipment.
Sharing HR not possible with all gym equipment
Some gym equipment use an older Bluetooth communication protocol than your Polar watch, and therefore it is not possible to share heart rate from your watch to those gym devices.