Captions that flash past are the accessibility failure nobody reports and everybody experiences. Check every cue against a target reading speed, ranked worst first, with the duration each one would need to come inside it.
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Measures how fast each caption asks a viewer to read, in characters per second, and ranks every cue worst first. For anything over your target it also shows the duration that cue would need in order to come inside it.
A caption that flashes past is not a technical fault. Nothing errors, nothing fails validation, the file plays perfectly. Some viewers simply cannot finish the line before it disappears. It is the accessibility failure nobody reports and plenty of people experience.
Fifteen characters per second suits younger audiences, dense material and viewers reading in a second language. Seventeen is a reasonable default for mixed adult audiences. Twenty-one is around the upper limit for experienced adult viewers on familiar subject matter. If your organization has a standard, use that instead.
Two options: give the cue more time, or give it fewer words. Extending only works if the following cue has room, so tightening the wording is usually the more reliable move, and shorter captions read better for everyone, not only for fast readers.