Flat pattern - SOLIDWORKS

Please for your appreciated help!

I got one position in STEP format that I need to convert as sheet metal, the position has a lot of radius. I manage to convert the position, but after the FLATEN order, only one small area "opens" to me, not the whole model. What could be the problem? I did not find a solution on YOUTUBE.

Many thanks to everyone for helping in advance!

Drazen