If the answer to the below is that the water level is below the sight glass and not coming up when you activate the fill button, then I'm afraid it will be for a service tech to sort for you.
First, determine if there is any water in the sight glass. It could be below (sight glass empty) OR it could be above the top of the sight glass (sight glass full and water level above it )? Either way, you wont see the miniscus in the tube and you will need to work out which it is.....
If the water level is already above the sight glass, and if the fill circuit IS working, you could simply be overfilling it more and more, and because it is above the sight glass, you cant tell......until it comes out the boiler safety valve....... Ergo, it could be that there isnt anything wrong, but if it is just overfull, you will need to bring the boiler level down to the right level (where it can be seen within the sight glass).
If you consider this line of thinking, you will be able to determine whether all is ok, or if it should go in for repair.
Hope that helps,
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