problem with blender

i crated a ceiling block for my FPS tyle half life , but some of the part of the model can’t see ,

example :

IIIIIIIIIIIIIII

IIIIIIIIIIIIIII


IIIIIIIIIIIIIII

???

THE FIRSt is the blender model , the second is the unity model , and the ??? , are the problem, i cant’ see middle model :-

any tip ???

Recalculate/Flip the normals in Blender under the tools section while you have the Object in EDIT Mode. This should fix the issue.