Due to being water bound, a float switch is always operating in an adverse environment. In a home sewerage system it comes into contact with household effluent including water containing cleaning products and treated sewerage. Because of this, the float switch is the most common failure, or most vulnerable component of a submersible pump.
Failure can happen if water leaks in, if it gets knocked, if it is operating in really adverse conditions, or if the wire inside the cable breaks. Wear and tear of the microswitch inside the float switch is another factor which contributes to its failure.