Options
1. Too much PG.
2. Too much flavoring.
3. A low-quality flavoring with ingredients like coloring, gums, sugars, etc.
4. Bad base (PG, VG, and/or Nic).
5. Contaminated bottles/syringes.
6. Oil-based flavorings in excessive amounts.
7. Wicking equipment.
8. Dehydration/other mouth issues.
I would isolate and check each of these issues and narrow it down.