Issues with Style Tool

There's probably a gap somewhere.  Try creating a fill surface that intersects the middle of the car.  Merge this to your existing model keeping only one half and then see if the remaining half will solidify.  If it does, keep moving the fill surface around until you locate the trouble area.  By the way, sometimes a cylindrical surface is easier to test with than a planar surface.

I often have same trouble. Best way for me is finding edge which have one side (they're connected with one surface).