Django set_password not working

I set the user password and it checks out, but I can't login in various login forms. I can't authenticate:

u = User.objects.get(username='foobar')
u.set_password('123')
u.save()
u.check_password('123')  # True
authenticate(username='foobar', password='123')  # None?!?!

I can't log the user in though I've tried various ways.