There's a curios bug/feature with external text subs which is difficult to catch if you don't know where to look. NMT players don't support subs in pure Unicode, only utf-8 or ANSI (and national variations like win-1251 for Russian). Open your subs in Win Notepad, choose File>Save As... and check what encoding is selected. It should NOT be "Unicode". If it is, select either "ANSI" or "utf-8" and re-save you subs.