subtitles move out of sync

Hi. I´m using final cut pro 7 and after making some simple editing corrections in my sequence, other subtitles further down the line become out of sync upon playback. They are in sync in the timeline, but when I play it back the subtitles start to appear before they are positioned in the actual timeline. This only happens after I make a change somewhere else in the timeline. Any clues as to why this is happening and how I can correct it? thank you very much