app preview disappears after changing the poster frame

Hey guys, I have a problem with uploading my app preview. This occurs every time I try to upload the app preview. I written down the problem in systematical text.

What I did with the app preview:

• Made my app preview in Imovie and compiled it in 4k resolution with a low quality.
• Seeing this article and followed the first answer: Unable to load app preview in iTunes connect

What works:

• Saving the app preview in iTunes Connect.

• Processing the app preview.

• Changing the poster frame from the app preview.

What not works:

• Saving the app preview again after changing the poster frame.

What happens:

• The app preview is disappeared, and removed from iTunes Connect.
• I never get any errors like most people get.

More information:

• Running Safari version: 9.1.2
• Running OS X version: 10.11.6

After trying and trying it worked one time.

What I did:

• In Imovie compiled the file in high resolution.

• Followed this answer: Unable to load app preview in iTunes connect

I think it didn't work at first because my file wasn't big enough. It was first 22 MB and now 74 MB.