stopped level up after Mastery Level 13
You need to spend Mastery Points (maybe needing to earn more to have enough) in a track, then you can continue that track.
Masteries work a bit differently to levels. You gain mastery points from specific achievements and finding points in HoT maps, not from XP.
XP is used to train up a mastery track. Once it’s full you need to ‘spend’ mastery points to unlock that mastery, which will give you a new ability, and then you start on a new track. (Don’t worry, points you’ve spent will still count towards your mastery rank, you don’t lose them.)
So if your XP bar is full that means you need to click on it (outside of combat) to open the Mastery window and click the button next to the track you’ve been training up to put points into it, then you’ll move on to the next one.
If you don’t have enough mastery points left over to do that you can switch to another mastery, if one is available, to start training that up. But you should also look at the achievements panel to see what you can do to get more points. All the achievements which award masteries have a mastery symbol next to it (green for HoT points, orange for Core points) and all the points are the same so you can pick whichever achievements you want to do to get the points.
“Life’s a journey, not a destination.”