[OG68]

The signal 'switch' doesn't have an earth. It just transfers voltage from the battery to the bulbs.

There is a flasher canister mounted on the lower dash to the left of the steering column. It has a bi-metallic reed that heats up when current is passed through and opens the 12 volt power source to the switch. After the reed cools down it closes the circuit and starts the process over again. It may need replacement or is of the wrong type.

There is a second canister mounted at the top of the fuse box. It is used for the hazard flashers and works on the same principle as the signal flasher.
The type of canister is dependent on how many bulbs are in the circuit. A heavy duty canister with a larger internal reed (higher bulb current to open the circuit) would be used for the hazard lights and for trailer towing. Used for a signal application, it may never get hot enough to open the circuit.

LED bulbs may also cause your problem. Since they are an electronic device and draw minimal current, there isn't enough current to open the canister reed switch. You will need an electric signal canister designed for LED bulbs.