Boiler filling loop issue

Until recently we've had no problem topping up our central heating via the filling loop. However now it seems the valve we'd usually open still turns but doesn't do anything. It doesn't feel particularly loose (there's still a little resistance as you turn it) but can't work out if our bathroom fitters adjusted something else instead.

Previously I'd just done a quarter turn on the top right valve - is there something I'm missing?

There both closed, you'd need to quarter turn the other valve aswell