There is a known bug for PGS subs when a BD is encoded to 720p: According to cccp, if the PGS subs are not resized from the BD default of 1080 to 720, some video players/renderers won't display them properly, meaning nowhere on the screen.

One fix for MPC-HC/CCCP is to enable the internal subtitle renderer to view these, then disable it again for other shows, as the internal renderer doesn't do as much as the external renderers, generally.  This setting is found in the Playback Options.  You'll also probably need to exit and restart after changing the internal renderer setting, and choose the full subs track.

Another fix is to have something like VLC installed to playback any BD shows you have that are 720p encodes with PGS subs, as they work fine as is with vlc.  You'll also have to change the subs track, as signs and songs is the default, I think.

These two are probably the easiest options.  tlacatlc6 mentioned a few other approaches in the chatbox on his website such as k-lite or manual additions and configurations.