Duplex night storage heaters - faults?

Hi I have a number of Duplex night storage heaters.
Most of them are ok but two have developed faults:
1.Does not store/charge off peak heat but does work when put on to heat imediately ie on peak!
2.does not function at all - this model does not have a "immediate heat" switch but just has ths overnight storeage and release dials.
Is there a common fault to these -any ideas?

David
January 2009