This issue sounds like a common Facebook Ads glitch where the video thumbnail shows, but the video doesn't play in the ad. Since it plays fine as a regular post, here are a few quick fixes:
1) Re-encode the Video
- Use a tool like HandBrake to re-export the video in MP4 (H.264, AAC) format.
- Facebook prefers 16:9 or 4:5, under 4GB, and less than 240 minutes.
2) Upload the Video Fresh Inside Ads Manager
- Don’t use a post boost or existing post.
- Upload the video directly into the ad creative section during ad creation.
3) Avoid First Frame = Static Thumbnail
- Choose a different thumbnail manually when setting up the ad.
- First frames that resemble static images can sometimes freeze the preview.
4) Check Browser Cache or Try on Mobile
Preview glitches can happen in browser. Check the ad on Facebook mobile or in the Facebook Ad Library.
5) Recreate the Ad From Scratch
Don’t duplicate a previous card-based ad. Make a new campaign, ad set, and ad.