Pull over that horse and stop questing. You might be about to lose some serious XP.
The most recent patch for The Witcher 3 seems to have created a pretty big problem - some quests are no longer rewarding players with experience points.
Speaking on the forums, one of the devs identified the bug, saying "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."
But forum members cited examples of completing quests less than 6 levels below their own and still not getting XP.
The devs responded, saying they had "found the issue and started working on a fix. We're especially sorry about this bug!"
According Polygon the bug is limited to PC and PS4.
Not everyone is being affected by the bug, but for now it might be best to stick to quests close to your level, and save often in case you need to backtrack and hand them in once the next patch comes out.
We'll keep an eye out for news on when we can expect the next update to iron out these kinks.