In cases where the connection drops between the HRM and the Phone or the connection is not reliable there may be problem with the strap.
You need to wash the strap with laundry detergents in a washing machine. Instructions here
If this doesn't solve the problem you can test the strap if you have access to a multimeter.
The picture shows where to measure. The resistance varies in different spots of the contact pad.
A working strap measures a resistance of around 1-4 kΩ (kilo Ohms).