I have set up some remote tracking branches in git, but I never seem to be able to merge them into the local branch once I have updated them with 'git fetch'.

For example, suppose I have remote branch called 'an-other-branch'. I set that up locally as a tracking branch using

git branch --track an-other-branch origin/an-other-branch

So far, so good. But if that branch gets updated (usually by me moving machine and commiting from that machine), and I want to update it on the original machine, I'm running into trouble with fetch/merge:

git fetch origin an-other-branch
git merge origin/an-other-branch

Whenever I do this, I get an 'Already up-to-date' message and nothing merges.

However, a

git pull origin an-other-branch

always updates it like you would expect.

Also, running git diff

git diff origin/an-other-branch

shows that there are differences, so I think I have my syntax wrong.

What am I doing wrong?