How can I force git-fetch to re-retrieve all the missing commits?

git-fetch completed with connection error messages (using http), and
subsequent operations to examine those remote branches gave me
error messages of missing objects.

While re-run git-fetch doesn't looks to work, since git-fetch thought he
has reached the most recent commit.

How can I force a re-retrieve of the missing objects??

-- 
Cheers
- eric