What could cause missing replies in an email thread?

My friend and I exchange emails and replies daily. When the text on the thread narrows we start a new thread. Lately she can't receive my replies and I have to start a new email every day instead of sending a reply. I get her reply to my email but she can't get mine. Also she can't get my original photos but she can get a screenshot of the photo I send to her. What is going on.? This doesn't happen with any of my other contacts. Help!