title: Unable to open pull request from fork
body: <p>I'm using Github to work on a project with a collaborator and we each have our own version of the project: I created mine by forking his. Several weeks ago, I finished with a set of changes, merged that branch with my master, and opened a pull request with my collaborator's master branch, which he accepted.</p>
  
  <p>Today, I finished another set of changes, merged the branch with them into my master, and am unable to open a pull request with my collaborator's master branch. We can't figure out why.</p>
  
  <p>Since I created the fork, he's made several changes, which I've merged into my fork by adding his repository as a remote and pulling from that. The project started as private and both of us recently made our branches public.</p>
  
  <p>Does anyone have any idea why I cannot open a pull request? I think it might have something to do with the fact that he had already merged a pull request from my master branch, and Github wants me to open pull requests from different branches instead of from my master, but it seems strange to me that I can't find any way to open a pull request.</p>
  
  <p>We were also thinking I could just create a new fork and copy over my commit history, if nothing else works.</p>
  
  <p>To clarify what I mean by I'm unable to open a pull request, I've attached some screenshots. From each version of the project, I can't find the other fork. It's as if the two were completely separate projects and not forks of each other. In both screen shots, notice how it says there are 0 forks, even though mine was created from his.</p>
  
  <p><a href="https://i.stack.imgur.com/AEWqj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/AEWqj.png" alt="enter image description here"></a></p>
  
  <p>From my fork, I can't select my collaborator's as the base fork (or as the head fork, for that matter).</p>
  
  <p><a href="https://i.stack.imgur.com/lwM7m.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lwM7m.png" alt="enter image description here"></a></p>
  
  <p>From his, I can't select mine as the head fork (or as the base fork, for that matter).</p>