You need to check if the model is not open at some point that "maybe" has forgotten with the EXAMINE GEOMETRY command, turn on all boxes and especially the BODY CHECK tab, if there is any point that the modeling is open the command will indicate in red color .
If this command does not indicate any problem you can use the command MAKE SOLID, if it is still not possible you can try a trick, UNSEW and SEW some face and use the EXAMINE GEOMETRY again.
(GOOGLE TRANSLATOR)