This is a common issue. The reason is most likely because the flat pattern derived configuration that has automatically been created is not flat. To solve this issue go back to the model, activate the flat pattern configuration and turn the flat pattern button back on.
Tip: If you want to toggle between the flattened state of the part, then activate the different configurations, do not select the flattern command as it causes issues like this when people forget. Does that make sense?
This is the derived configuration that it creates when you create a drawing from the part.
Not sure which version it started to act strange but thinking maybe 2011, not sure.
Back then I figured out that when you insert a sheet metal part into a drawing template, the insert dialog box pops up which allows you to choose which view you want to show. I found that if you always check the "Flat Pattern" view last the flat pattern would come in correctly, however if you were to pick that one first it would act like you mentioned, and when that happens open up the part and do what Philip Downing mentions..
If you can't figure it out - attach your files here and end all doubt.