a. collar batteries are depleted: recharge
b. control unit is removed from mains: check
c. collar not fitting snugly enough: One finger should separate probes from coat. Ensure long-haired breed probes are fitted for those breeds.
d. Collar needs to be reset outside of active zone. Use magnet on recharging leads to reset collar.
e. Boundary Loop is buried too deeply: ensure it is not more than 10cm (4 in) below ground).
f. Boundary Loop is connected to wrong connector on Control Unit. Ensure 500m+ loops are connected to the 500m+ connector, and the return section to the earth connector.