The gum line around the crown is always sensitive and flares at times.

Hi:The first thing that should be checked is bite and clenching and grinding pressure on the crown.  If the crown contains any metal there could be metal allergy.Best,Dr. Maddahi

Depending on many factors, at the gum line where the crown meets the tooth there can be a slight ledge or a large ledge. There will always be an area that is not as smooth as your tooth is without a crown. This area must be cleaned better than you clean around your natural teeth. The gum must have floss run underneath it daily, be brushed aggressively and also stimulated with a rubber tip. Also be sure to have your teeth cleaned in a dental office every 4 to 6 months. This routine will keep the inflammation down around your crown.