may gas oven wont stay on (belling G757)?

so to ignite the oven you have to turn the knob and push it in for the gas to cum through, then push the ignite button, keep the knob in for a few seconds, release and the oven stays on. it all works fine until i release the knob then the gas stops cuming through the pipes and my oven goes out. my grill works in the same way but thats fine and all my burners on the top work fine too. please help or it looks like beans on toast for christmas day :-(