Why has the enamel crawled in from the edges?

Why has the enamel crawled in from the edges? This is because the piece is almost certainly over fired (example right); to combat this problem you must reduce the temperature and/or the firing time. If you are using a torch for firing, the flame has enveloped the piece. Try holding the torch further away so that the flame hits the back of the blank and forms a 45° angle.