[Cura leaves parts out?]

hello all,

 

Im kinda new to 3d printing, I use Cura as my slicer. 

I got a file from a friend of mine he wants to get printed on my printer. only when I import the stl file to Cura it look fine (some errors but non important)

But when I slice the model it leaves very big parts out?

 

thanks very much guys!

Alexander Mosselaar