There can be many reasons that subtitles cannot be displayed.
easyDCP Player performs a whole range of checks when it loads the subtiteles.
It will automatically verify that the XML document´s structure meets the specifications and it will also make sure that all characters used in the subtitle document are actually present in the enclosed font file. (Font)
If any potential issues were identified, they will be shown in the log window.
In short, make sure no warnings are raised when the DCP is opened by eayDCP Player.