If you see white specks in your enamel, remove them. This is a sign of deterioration. Some times there is only a few and you can remove them before you place any enamel in your work. Sometime the enamel sits on the bottom and water will not penetrate the enamel at all. This it time to throw them away!
If you did not see this until the enamel is fired you can drill it out with a diamond bur and re-fire the piece before adding more enamel. This way you will not have a shadow where you removed the speck.
This is a real issue when using transparent enamels, an it comes from the enamels being left wet too long.