You don't say, but this is probably a PAL vs NTSC issue.
From Joe Bowden in 2008: "NTSC subtitles must use the semi-colon ( ;) delimiter because it's drop-frame. PAL timecode is not drop-frame so it always uses the colon ( :) delimiter."
Found his quote before the recent thread on this; couldn't resist quoted it here.
So your text file may be set wrong; or you have not adjusted it to account for the framerate; or both.
