I used to be able to mark messages unread and it stopped working

Thank you for the reply marafromchicago!

Let's make sure your device is up-to-date. If it is updated, let's restart your device. Restarting your device and making sure it is updated can resolve many unexpected issues. 

Take a look here on how to restart your iPhone: Restart your iPhone - Apple Support

Take a look here on how you can update your iPhone: Update your iPhone or iPad - Apple Support

Keep us posted!

Hi marafromchicago,

We'll be glad to help you out with this. If you're referring to using this feature with the Messages app, the following article shows how to do this: Keep track of conversations in Messages on iPhone - Apple Support

"Mark messages as unread

You can mark conversations as unread so you can return to them later when you have time to respond.

In the Messages list, swipe right on a conversation to mark it as unread. Or drag right and tap .

Tip: You can also mark several messages as unread at once. Tap Edit, tap Select Messages, select the conversations you want to mark as unread, then tap Unread in the bottom-left corner."

Cheers.

It should be working on your iPhone, but if it's not, you can contact Apple Support by calling or texting (800) 692–7753. There is no other way, your phone probably has a bug that cannot be fixed unless you call or text Apple.