Why do Streaming-Services only let you "Skip Intro" Sometimes?

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I watch a lot of television-series on streaming-services (SUPERGIRL, LEGENDS OF TOMORROW, MONK etc. on hulu, Amazon Prime & NETFLIX). Those shows feature introductory footage at the beginning of every episode, and the streaming-services often let the watcher "skip intro" (indicating you can 'press the select-button on your remote' and the streaming-video will instantly fast-forward to the end of the intro). But quite often--even when the intro-footage is still the same as always--the service doesn't give us the "skip intro"-option Why not?