A fixed offset can correct a constant timing gap; it cannot necessarily correct a track whose timing increasingly diverges from the programme.
Why this happens
There are two useful timing patterns. With a constant offset, subtitles remain about the same amount early or late throughout the programme. With drift, they may begin close to the dialogue and move progressively farther away. Treating the second pattern as the first leads to repeated adjustments that never stay right. The track may belong to a different edit or timing basis, or the playback route may be handling its timestamps poorly. The visible pattern matters more than guessing one universal cause.
Compare a line near the start with one much later in the same item before saving a global delay. A small supported offset is reasonable for a constant gap, but a track that matches at one point and not another deserves a different track or edition comparison. Avoid treating a manually chosen delay as proof that the whole subtitle is correct. Keep the two approximate positions and whether the gap grew or stayed fixed. That gives a clear account of the problem without requiring somebody else to guess what happened between repeated slider changes.
A useful way to work through it
Compare subtitle timing near the beginning and later in the same item.
Use a small offset only when the gap stays roughly constant.
For drift, compare another available track or matching source edition.