I've got both - H7 and H10 HRM from Polar and 935 won't connect or even pair sometimes...
I've found the issue in my case - it will not pair and connect if my phone's BT transmission is ON, if i turn it OFF - them pair/connect/transmit HR data wonderful!
So, what it is - either they paired with the phone (Android Sony Xperia Z2) or 935 connected strongly and one-to-one to the Sony - don't know! But Now if I want to start connection - I turn off Sony's BT for a while and after connection - turn it on back...