Ditto the check the port for impacted lint. Embarrassingly had that happen to me when took my old 6 to Apple Store thinking something else going on: Genius took his SIM ejector and pulled out a giant glob of lint. The old 30-pin connectors were more obvious when this would occur, so, made for "easier" self diagnostic.
Once a month, I take some canned air to the port and the speaker holes (those might be more obvious to get a guess on lint in the Lightning port). Also a soft (old usedj toothbrush across the bottom to help get stuff out.