Hi,
Please make sure that the mimetype file is not compressed and is the first file in the archive.
The command line way to do this is:
zip -X0 "full path to new epub file" mimetype
then followed by
zip -rDX9 "full path to new epub file" * -x mimetype
This is effectively what the epub Zip for Mac OSX tool does:
More info can be found here:
https://www.mobileread.com/forums/showthread.php?t=55681
I am sure there must be something that does this automatically for Windows but being a Mac user, I don't know the name of it offhand.
Sorry I can't be more help.
Kevin