The dull spots are your answer hiding in the question: WATER SPOTS. Any moisture left in a cavity marks the shell cast against it - the mould was clean, just not DRY.
The routine: wash in warm water (mild soap if needed, nothing abrasive - scratches transfer to every future shell), then dry completely with tissue so no droplets or mineral spots remain. Then the professional step: polish the cavity interiors with cotton - a cotton ball worked in circles - which buffs the surface to the mirror finish your shells inherit. Cover the polished mould with a cloth until use so dust doesn't resettle.
Handling rules for their long life: never metal tools inside the cavities, never stack unprotected, never chase a stuck shell with force (proper temper releases on its own - the mould is innocent). Treated this way polycarbonate outlives most marriages 🙂
