My name is***** am sorry to hear that you are having this issue.
I will be glad to assist you with this. Are you using the Gmail app?
You can reply to me here by typing in the box below please.
If yes, follow these steps please>
In your email list, open up the email that you want to mark as unread.
The emails with bold titles are unread and the ones with regular font are read emails.
Then tap the arrow in the top right-hand corner of the screen to bring up a new window.
Then tap on the envelope which is closed and says "Mark as Unread" and tap it once.
When you tap on that option, the email will be marked as unread.