Watch this video by MSQC:
https://www.youtube.com/watch?v=0vCWpxBRs20
At 6:40 into the video, she will show you a different way to stitch the corners. I saw Patric Lose demo this on F&P Love of Quilting show (on PBS). The other thing that will improve your corners is to make sure the small fold on the back is laying in the opposite direction from the fold on the front. This will disburse the bulk evenly in the corner.