If you have a Cad Manager who handles udating the printers and templates I would ask them first.
In order for 2015 to be able to find the ctb file it needs to be mapped to the folder location containing the ctb file in OPTIONS.
From within 2012 (if you still have it installed), enter STYLESMANAGER @ command line.
This will open Windows explorer to the file location where AutoCad is currently finding the ctb files.
In 2015 version enter OPTIONS @ command line > Files Tab > Printer Support File Path > Plot Style Table Search Path > Specify the folder containing your ctb files here.
Also verify the Printer Configuration Search Path and the Printer Description Search Path's are looking where they should be.