3D Tetra Elements not showing in LS-PrePost/DYNA

Hello,

I imported my geometry into HyperMesh and used automesh to generate quads on the surface.
I then used volume tetra to generate 3D tetras and then deleted the 2D elements on the surface.
When I export my model to DYNA keyword file and open it in LS-PrePost, it is missing the extra pyramid elements at the surface.

E.g. 2D view in HyperMesh:
 

View of surface using mask option in HyperMesh.

Surface in LS-PrePost:


Any thoughts on why this is? How can I retain the elements at the surface?

Thanks,

Petros