If the communication fails, check that
the infrared devices are positioned correctly. The infrared windows should be within 30 degree angle, no further than one meter (39 inches) and no closer than 1 cm (0.4 inches) from each other.
nothing is blocking the path between the infrared windows of the devices.
there is no other infrared device nearby.
your Polar wrist unit is in the connection mode.
the battery of your Polar wrist unit does not need replacing (i.e. low battery symbol on the display).