Special characters changing to "?" using ArcMap

I am using shapefiles in Spanish that I’m using to build a database and the special characters are displayed as normal, but as soon as I ran an operation on the layer these characters change to ?.
I tried using field calculator to replace ? with them but as soon as I save the edits it reverts back.

I’m using ArcMap. When I added the shapefile the attribute tables had the special characters without a problem byt when I preformed an operation such as Merge or Dissolve or any other analysis that would deal with attributes, it would change every special charterer to ?. What I tried doing was using field calculator and replaced the ? with ñ for example and it worked but when I saved the edits the ? would return back as if I didn’t replace them.