What I would try is another coat of mud and feather it out farther on both sides of the seam. Probably at least twice as much as it is now and be careful not to sand to hard.
I did this after priming the joint with Kilz. It reappeared as it dried. These cracks are in a butt joint on the ceiling by the way. Another area I just put about 4 coats of Kilz on and at the moment it is all but invisible but it is like a ticking time bomb probably.