Yes, I was in Expert Mode. What I mean is, in previous versions of PSE, there was an Interactive Layout option among the Photomerge options - in other words, if two photos did not match up closely enough to merge them, you could do it manually.
For some strange reason, PSE 12 does not have this option!
A site like Photojoiner allows you to do this for free, so it seems crazy that Adobe had it but removed it.
Now, I am having to go to Photojoiner every time, which seems anuisance.