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Is this Lightmap issues? Maya > UE4

Hey guys, first time poster and I am relatively new to this awesome software. So I was hoping for some advice.
I have been using Maya to build up the city I live in for a while and decided to see if it could be brought into UE4.
I have exported a couple of buildings. Some seemed OK. Others, well i realized I have likely lightmap issues.

The process in which I create the second lightmap in Maya is to duplicate the first texture, then hit automatic, then chose back facecing, press flip, export as FBX and bring into UE4.
I set the UV lightmap resolution to 1024 and whilst the majority of the building looks OK (for a newbie) as you can see in the image there are certain faces that are not light.
When I checked in the Maya lightmap UV to see if the particular face is backwards or overlapping, it seems not to be.

Some of the other shop fronts are worse.

Any ideas what I might be doing wrong>? Thanks very much for your time.