Boss therm bps242rf timer not coming on?

Hello I have the boss therm bps242rf and have my timer on but last few days the heating is not coming on the small triangle that comes on the display when heating it meant to be on is not coming on so I have to toggle the switch along and back along to automatic for it to come on anyone any ideas as to why it doesn't seem to recognise when it's meant to be coming on. Thanks