This is oftening problem when for model get lost smoothing settings. For fix it situation have several solutions.
Edit Poly / Edit Mesh
If you add "Edit poly" modificator on the stack, smoothing group problem can be resolve. Use modificator "Edit Poly" for editable poly and modificator "Edit Mesh" for editable mesh. Need use exactly modificator from modificator list, don't item "Convert to" from context menu. After it smoothing (tool Smoothing groups) will work and you also can convert model in editable poly or editable mesh to merge modificator with model.
For solved this problem you can do following: Apply an "Edit Normals" modifier, select all normals (Ctrl + A - select all), then press "Unify" button in modificator settings. This should fix that. Collapse modificator stack.
Also you can reset the XForm on model for guaranty.
Also you can use modificator "Shell", but it also change geometry.