965 HR sensor fails when swimming

Hi,

I set my 965 to my wrist as usual. I identified that I have to tighten the band a bit for HR sensor to be more correct in the water. However I feel that armband is correctly tight to the skin.

During swimming often it goes down to 60-80 pulses per minute, sometimes it takes 10-15 minutes to go up and identify reasonable pulses (140-150).

Thanks for support