I agree with Holice. It sounds like the problem is the way the corners are being made. Here is a video on how to make the mitered corners that may help. I haven't watched the entire thing so I'm not sure if it shows the corners being turned, but if the corners are sewn correctly they turn very easily when you are ready to sew the other side of the binding. Hope this helps.
http://www.bing.com/videos/search?q=...ail&FORM=VIRE1