Missing Faces when importing mesh in engine

Hi im confused as to whats going on, in maya it seems to be alright no holes no nothing, but in engine u can see a few,
even retopping again creeated some other holes in other areas like it aint consistent if anyone could help me on this
that would be appreciated, like its purposely deleting these, also, when exporting i got to check the bi normals and tangents or things go black and weird if that helps the case.
can provide the mesh too