Sync ms audio on a movie gets desync

When I sync the audio on a movie at the start its perfectly sync but after 15-30 minutes sometimes with just 5 minutes the movie gets desync again, why? There is no subtitles, no timestaps on the video, nothing, just the video and one audio so I dont know where is the problem. Happens with every movie I try to sync. The audio difference is no more than 20 seconds.

I had no problem with one movie where the audio was only 2 seconds difference.

Okey it was the fps audio, Ive used meGUI to convert the audio to the same movie fps and thats it, no more desync.