On Sun, 28 Jun 2020 22:54:20 +0100, John Rumm wrote:

So either a lockout / fault condition preventing it proving and firing,
or something like a valve problem - either not getting an instruction to
open, or not responding.

Or the boiler is controlled via a microswitch on a valve and the
microswitch has failed or the valve is responding but is gunked up
and not opening enough to operate the switch.

--
Cheers
Dave.