Cannot attach an e-mail attachment to a new e-mail message in Outlook

Hello,

I have a couple users trying to attach an e-mail to a new one and send it out, however, when dragging it from the e-mail list to the new e-mail window, nothing attaches. This doesn't occur with all e-mails, just a select few. 

Any obvious reasons on why this would be? When I inspected the e-mail they were trying to attach, nothing seems abnormal and the file size was small.