Sounds like whoever did the drywall job did not apply enough coats of mud on your corner (same thing happens at joints sometimes also where 2 pieces of drywall are taped where they line up against one another). My standard is 4 coats of mud need to be applied to all corners, joints, etc. prior to the final sanding job. Unfortunately, many drywall subs these days will only throw on a couple coats of thin mud and then you end up with problems like you're describing. Seems to me your best option at this point would be to re-sand the corner and re-apply a few (3?) more coats of mud, allowing 12-24 hrs. drying time between each coat. You will also want to lightly sand each coat after the drying time to smooth the coat out and get rid of any visible ripples in the dried mud. When you sand your final coat, the mud should be smooth and slick and uniform/even with your wall. Then, you guessed it, re-paint the drywall. Hope this helps....good luck!!