Poster image on video not displaying on ipad

[Apostrophe]

I've got an mp4 video file embedded in an InDesign file (CS5.5), I've selected these options: play on page load, [loop is unselected], poster: from file, controller: skin all over.

The mp4 has been exported from MPEG Streamclip, following these options:
1. Open MPEG Streamclip and drop your content into the program
2. Choose File > Export to MPEG-4
3. Make sure the following options are selected:

Frame rate: 24
Frame size: Select the option that displays with "(unscaled)"
*Please note that the iPad is capable of frame sizes up to 1920x1080 and the iPhone is capable up to 640x480. If your frame size is larger than 640x480, Blurb suggests selecting either 480x360 for (4:3) content or 640x360 for (16:9) content, to ensure successful playback on all iPhone and iPad devices.
Quality: The higher the quality, the larger the file size. Experiment with what looks best inside your ebook, but the minimum should be at least 50%.
Interlaced scaling: Uncheck this option

I'm converting it in Calibre 10.4 (which I've recently updated from the previous version).

When the ebook is viewed on the ipad, one mp4 video shows the poster image, and the other doesn't.

Can anyone help?

Thanks!