Invisble/flipped/missing faces on export

stubins

I'm a newbie to rFactor and have a question regarding exported geometry. When my geometry loads into rFactor some of the faces don't show up so I'm not sure if they're missing/invisible or flipped? Has anyone else come across this issue?
Just to explain it further, say I have a box that is all one mesh then the top and front polys are missing. I don't understand as they have the same material and all of the vertices are welded, I've even tried flipping the faces in 3Ds Max but they still don't show up.
Is there a fix and a reason for this please?
Thank you in advance for reading this.
 

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