Bluetooth is a short range wireless technology. It's range is 8-10m line of sight. It is affected by obstacles such as water, wall, metal, furnitures, other wifi signals etc.
Human body, which mostly is water, affect the signal heavily. So 'Cross Body Issue' is still a challenge for Bluetooth products.
Best result can normally be achieved by keeping the transmitter (phone, PC, etc.) at line of sight with the antenna of the bluetooth device (normally on the main button side). If you want to keep the phone in pocket, then you are suggested to keep it on the same body side with the headphone antenna to avoid the cross body issue.