Hi Mark,
Its hard to say, Sand blasting creates a rough profile on the surface and a larger surface area for the powder to cling to. The more aggressive the profile the larger the surface area but also the rougher the finish. Chemical stripping works well also but the part should be washed and cleaned very well. I like to use a scotch brite pade while I wash the part. It helps lossen up evey thing so I can rinse it away.
Your right though it shouldn't chip that easy. Two things come to mind Cleaning and Time at Temperature.
I don't know what process was used to clean the metal. Is it steel or Aluminum? It must be absolutely clean. Just the touch of your hand will leave an oil residue that may cause defects in the finish or cause lifting in places. Using a solvent based cleaner will also cause adhesion failures.
One of the most common problems is a failure to get the substrate (base metal) to the correct temperature. (Not long enough in the oven) Recommendations on the powder box may say something like 398F for 10 min. That's ONLY the time required to cure the powder. The base metal of the entire part must also reach that temperature for proper adhesion. The powder may have cured properly but failed to adhere to the metal.

Mark