I had the same issues with DFP 2.2 and found the following fix to the issue.
Double click the DFP properties in DW Bindings Tab
Select Next to go to the area that allows you to assisgn upload folder etc.
Click the lightning bolt next to Default File and select you hidden field from the form.
Now cut the code that has been inserted and paste it back in the same spot. The parameters will now bind correctly. If you don't do this then whatever you select using the lightning bolt will not retain in this field.
I only noticed this when i went back in to change a parameter and noticed the field was blank everytime i returned.
Hope this helps!