I have a gas fireplace in my living room & I’ve had to turn it on without and problems. It’s come time to turn it on again & when I go through the same steps, it’s not working. I have a knob that’s marked off, pilot & ON. I turn the knob to pilot, light the pilot & hold the knob in for approximately a minute. When I release the knob the pilot light stays lit (great success) but when I turn the knob from pilot to ON, the pilot light turns off. What am I doing wrong/what could possibly be broken in this situation? Thank you in advance!