Boiler not kicking in when stat calling for heat ( intermitt

[petebilt53]

Hi I have Evohome single zone home pack installed, at times when turning the stat to call for heat ( which the stat indicates ) there can be quite long delay in the combi boiler kicking in, the same happens when increasing the room temp from the app. This can be overridden by pressing the button on the relay box, which would kick the boiler straight away.
Your advice would be appreciated.
Thanks Pete..