There are several possible causes for enamel flaking off the copper.
However, the main cause might be with the copper itself. For
enameling, it is recommended that one use oxygen free electrolytic
pure copper such as sold by Thompson enamel. Many do use plain
roofing copper with no problems, but I prefer not to take any
chances as In addition to enameled jewelry items, I do large scale
wall enamels, and the last thing I would want to happen is to have
some of the enamel flake off, possibly weeks after the item was
Many years ago, Woodrow Wilson addressed this matter in one of the
articles appearing in Glass on Metal. He pointed out that when using
plain roofing copper, there was a possibility of enamel flaking off
months after a work was finished.
You mentioned doing a thorough cleaning of the copper, using a
copper cleaner. After using any copper cleaner, it is advisable to
neutralize thepiece thoroughly, before enameling.