The self-cleaning feature on my electric, free-standing, ceramic top oven has never worked. I don't use the oven a lot, but am now in dire need of cleaning it. I press the self-clean button, it brings up the time on the display and flashes Set. I press Start and it says the door is locked (this flashes) but after about a minute it beeps and then goes back to the clock and the door unlocks. It is all electronic display and locks automatically, not manually. Other than this, it works fine. Any thoughts?