Fairly common that the fill matrix behind the left hand side panel gets a limescale blockage around the internal, I think, blue comma shaped insert you should see through the semi transparent unit near the top. Remove and catch any water with a towel as being removed, turn it upside down and try to blow down where the valve pipe`s nozzle was fitted, any restriction is the cause. The cure is to buy a bottle of Kilrock descaler and pour SOME of it down that nozzle with a dry cloth or wear rubber gloves and if you look very closely you should see it fizzing = limescale present. leave in a w/up bowl for 5 minutes, drain out the Kilrock, clean nozzle of acid residue and blow rhtough to test. Be very careful of Kilrock: eyes mouth & fingers, beware, nasty stuff but this is how I clean out limescale from these matrixs.