Possible causes are

1.   Wires are not properly secured or connected in the terminals. Make sure the wires are secured, screws are fastened and the Live or Load wires have been connected to the correct terminals.

2.   The Consumer Unit MCB in the property is OFF or has been tripped. Switch on the MCB. If a breaker continually trips, contact a registered electrician to find out the cause of the problem

3    Temporary disorder. Switch off the MCB in your consumer unit and then on again to reset the switch

Ulti switches / dimmers are now discontinued.