Hi Cakey, I am sorry to hear that Anna is still having problems I do not have specific information on your machine and I do not recall the full story of your previous problems or the components that have already been replaced. Are you sure that the pressurestat and heater relay are in good condition?
I do not think that there is any logic to prevent the heater turning on during a shot but there is logic to prevent the heater turning on if the boiler water level is too low. In these circumstances the pump should run (as you are pulling a shot it will already be running) and the boiler fill solenoid open to refill the boiler. An intermittent problem with the boiler fill solenoid or its coil or the controller could result in the boiler not refilling and hence the heater remaining off.
I will keep my fingers crossed that the new solenoid/coil fixes the problem