why does my export project (mp4) play only the first 10mins when it’s an hour long?

Do a Control-click/Get Info on the file and check how many GB it has. One hour long should be at least 15 GB. If it is much less than that then the export didn't complete. In other words, do you have only 10 minutes of file or do you have one hour of file of which only 10 minutes is playing.

-- Rich

Hello and welcome to Apple Support Communities apple_001,

I see that you're having an issue with a project that you've created with iMovie not playing all the way through. I understand your concern, and I'd be happy to get you pointed in the right direction.

Take a look at the following article for iMovie. If after taking a look at the following article, the issue still remains, you'll want to reach out to Apple Support.

iMovie Support

Contact Apple Support

Thank you for using Apple Support Communities.

Enjoy your day!