Quick answer
Use an MP4 video encoded with H.264 video and AAC audio — it's the format PowerPoint plays most reliably on Windows and Mac. If a clip still won't play, run File → Info → Optimise Media Compatibility, and keep any linked video in the same folder as the presentation.
WHY IT HAPPENS
Wrong format, or a link that broke when the file moved.
Two things cause most playback problems. The first is format: PowerPoint can only play certain codecs, and a video saved as an odd type — or an MP4 using an unusual codec — triggers "codec unavailable" or simply won't start. The second is linked media: if a clip was inserted as a link rather than embedded, the presentation only points to a file sitting on your computer. Email or move the presentation and that link breaks, so the video vanishes on the other machine. Both are quick to sort.
THE FIX
Work through these in order.
- Convert to MP4 (H.264 + AAC)This is the format Microsoft recommends and the one PowerPoint handles best across Windows and Mac. For audio, use M4A with AAC. If your clip is a MOV, WMV, AVI or an unusual MP4, convert it with a free, reputable tool like HandBrake — choose the MP4 container with H.264 video, then re-insert it.
- Run Optimise / Compress MediaGo to File → Info. If PowerPoint spots media that may not play on other devices, an Optimise Media Compatibility button appears — click it. Just below, Compress Media re-encodes and shrinks the clips, which both improves compatibility and makes the file smaller and easier to email.
- Fix broken linked mediaIf a video plays for you but not for anyone else, it was probably linked, not embedded. Put the video file in the same folder as the presentation before you insert it, then insert with Insert → Video → This Device. Keeping them together travels far better than a link to a folder only your PC has.
- Turn off slide show hardware accelerationOn some machines the graphics card causes video to stutter or show black. Go to File → Options → Advanced, scroll to Display, and tick Disable Slide Show Hardware Graphics Acceleration. Click OK and try the slide again.
- Install the missing codecsIf PowerPoint still can't play a format you need, your PC may be missing the codec. A well-known pack such as the K-Lite Codec Pack adds broad support — but only install codec packs from trusted sources, or ask your IT provider, to avoid bundled junk.
- Update Office and WindowsPlayback bugs are often fixed in updates. Open File → Account → Update Options → Update Now in any Office app, and check Windows Update. Restart afterwards, then reopen the presentation.
- Test as a saved copy, not from emailPlaying a presentation straight from an email attachment or a still-syncing OneDrive file can block media. Save it to a local folder first, open that copy, and test playback there.
Presenting to a client or on a big stage? Do a full run-through on the actual laptop and screen you'll use — with the sound on — well before you're due. Most "the video didn't play" moments are a format or link issue that a five-minute test the day before would have caught.
