Water heater dump

[Jammi]

Morning All, I’ve just picked up the van from the workshop after having some work done on the water heater.
As it’s freezing here I’ve emptied the tanks and turned the blue switch to dump any water from the heater, but the blue button isn’t popping out.

Does anyone know if this could be a problem? Does it also mean it won’t empty automatically?
Is there a trick to loosen it if it’s stuck?

Thanks in advance.

 

[Jammi]

Or will it not pop out if there’s no water in the boiler?