The most recent patch for The Witcher 3: Wild Hunt seems to have brought with it an unfortunate side-effect, and some players are finding that completing certain quests is no longer granting them XP.
One dev acknowledged the bug - which according to Polygon is only affecting the PS4 and PC versions of the game - in a forum post: "Quests that are 6 levels (or more) lower than yours are not granting experience. Anything else should reward you with exp, we are looking into the issue."
Players, however, have been able to cite examples of situations where missions closer than the six level divide mentioned by CD Projekt Red are causing the same issue. Either way, the developers have found the bug and are working on a solution.