Does it render ok? If so, then the problem is likely to be a combination of the Scene Scale (system units), distance from the origin and how far apart the faces are. 2 faces which may be a foot apart are effectively coincident when viewed from hundreds of yards away.
Booleans, unless used very carefully, can create really messy objects. If that's the case, then it may be quicker to recreate the object using the existing one as a guide. Trying to clean it up may take longer than recreating it, but that does depend on the complexity of the model.
Max 2016 (SP1/EXT1)
Win7Pro x64 (SP1). i5-3570K @ 4.4GHz, 8Gb Ram, DX11.
nVidia GTX760 (2GB) (Driver 430.86).