Why are some iPhones unable to play MP4s?

Notice that macOS 15 Sequoia and earlier QuickTime Player does not play video codecs like HEVC/H.265 with Codec ID hev1.

For example ffmpeg by default uses incompatible HEVC Codec ID hev1 so take care to use Codec ID hvc1 instead when encoding. Or convert to Codec ID hvc1:

ffmpeg -i input.mp4 -c copy -tag:v hvc1 output.mp4

Use 3rd party apps like MediaInfo or Invisor to check such pesky details:

MP4 is a wrapper. The audio and video inside are individually coded using codecs which come in many varieties. It could be that your app is using a proprietary one or just one not included in Apple's decoding software. Try using the free VLC to play the movie if this is not a protected file.

Since you are using Windows it could be you are indeed using a codec that is not Apple-friendly.

I also invite you to browse the vendor's own support page https://support.techsmith.com/hc/en-us#kb

Some more details:

• https://www.captureguide.com/iphone-video-format/

Here are what Apple recommends for media creation, when submitting that media to Apple for distribution:

• https://help.apple.com/itc/videoaudioassetguide/en.lproj/static.html

If you’re picking one, then HEVC / H.265 works with iPhone 7 and later, iOS 11 and later, and macOS 10.13 and later, and it is very efficient:

• Using HEIF or HEVC media on Apple devices - Apple Support