Worcester 35cdi 2 boiler hot water from some taps not from others ?

Hi
I have a Worcester 35cdi 2 boiler that is acting up

Some hot taps fire the boiler and produce hot water
others don't.
I figure it's to do with flow but when I look online they sell a flow switch for 35cdi but not 35cdi 2
when I look in installers manual there is no mention of flow switch but a flow sensor ?
is this what I need to change ?
it says reinstall using heats ink paste (like a pc cpu)