I was making a cylindrical sheet metal.  I tried several methods online (including creating base flange from sketch and converting an extuded part to sheet metal) but all I got was this weird result.  The bounding box is very small so when I flatten it, I only have a small piece left.  How come? I followed every step from some youtude video.

Thank you very much!

btw, I am using SW2012.  If I use 2013 then there is no such problem