No subtitle to select when playing mkv format

[toppertje]

Hi,

maybe there`s someone here who can help me.

I purchased last month the 42PFL6877.

When I want to watch a movie (.mkv) which is streamed from my pc, I cannot select any subtitles (.srt).
Both files (mkv and srt) are placed in one map, same titles and everything.

When I replace these two files to a external drive and connect it to the tv by usb then I can select
the subtitle and everything is working properly.

I don`t understand why the tv cannot handle both files when they are streamed and why can I select the subs
when I watch the movie from the external drive which is connected by usb..

Please help me and give me the solution..

thanks in advance

grtz from Holland!