Most of the time the video/movies provided on the internet was 25 fps while common subtitle fps are 23 fps or 29 fps. This will cause subtitle delay, no matter how many times you adjust the time synchronization the subtitle will keep delay over time. You can check your video framerate by right click on it and select properties-details.
So here is how to solve this problems:
1. Download SubtitleEdit-3.4.6 from www.nikse.dk/subtitleedit
2. Select the standalone package, download it and extract the zip file.
3. You can also download the installer package if you prefer to install it in your computer.
4. Run the program and load the subtitle you want to change the framerate.
5. On the top panel select synchronization and select change framerate.
6. Now key in the new framerate and save the subtitle file.
The common subtitle framerate is 23 fps for DVD and 29 fps for bluray. Most of the time I just test until I got the right value.