Avoid the microwave and the oven in heating the ware, especially if the pot is cold. I consider Majolica to be best used for serving and dining, and not for use as cooking pots. Fired glaze has 'white spots' in it. Another common cause is that the surface of the piece was too rough, and needed to be burnished after trimming to remove the 'grog marks' left by the trimming tool. Glaze 'crawls' over certain areas.
Probable cause is too abrupt a change in form.
Where a footring meets a bowl, for example, the area should be curved, not angular. Wherever your form changes direction abruptly, curve the juncture. Crawling can also result from applying the glaze too thickly, or applying the colors too thickly, especially the EZ colors. Glaze 'shivers off' during cooling. Cause is that the edges of the form are too sharp or thin.