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Gas stove repair in our kitchen

We have a Classique company gas stove at our rental property. One of the main hob's knob doesn't seem to be working. When we press and turn the knob, hob is working fine. As soon as we release the knob, flame is going off. For some reason, knob is not holding there. Could someone please guide me your rates on fixing it and what could have gone wrong here? I want to get it fixed this week if I can and it is depending the costs involved.