If you look at the shaft onto which the knob fits, there is probably a flat bit. That engages with a springy insert in a wee rectangular cutout in the back of the knob.
The springy bits often seem to get soft and lose springiness. If it's not dropped out and been lost completely, it might be possible to bend it a little to make it a tighter fit.
If the thing is beyond it, many suppliers of cooker spares offer replacement knobs, either model specific or "universal".
Try espares or an outfit called RanXsoms (remove the X. Why? Howtomendit.com seems to object to certain strings of letters)