My Ariston gas oven will not stay alight. The burners and the grill (in the top of the oven) are fine. When I turn the oven knob to "on" and press it in for the ignition, the gas comes out, the ignition sparks, the gas burner lights. But as soon as I release the knob the flame goes out. What could be the problem, please? Thank you. clivesimmonds07@gmail.com