It sounds like your problem does not involve mocks, but I just spent all day debugging an issue with similar symptoms, and your question is the first one that came up when I was searching for a solution, so I wanted to share my solution here, just in case it will prove helpful for others. In my case, the issue was as follows.
I had a single test that would pass in isolation, but fail when run as part of my full test suite. In one of my view functions I was using the Django send_mail() function. In my test, rather than having it send me an email every time I ran my tests, I patched send_mail in my test method:
from mock import patch
...
def test_stuff(self):
...
with patch('django.core.mail.send_mail') as mocked_send_mail:
...
That way, after my view function is called, I can test that send_mail was called with:
self.assertTrue(mocked_send_mail.called)
This worked fine when running the test on its own, but failed when run with other tests in the suite. The reason this fails is that when it runs as part of the suite other views are called beforehand, causing the views.py file to be loaded, causing send_mail to be imported before I get the chance to patch it. So when send_mail gets called in my view, it is the actual send_mail that gets called, not my patched version. When I run the test alone, the function gets mocked before it is imported, so the patched version ends up getting imported when views.py is loaded. This situation is described in the mock documentation, which I had read a few times before, but now understand quite well after learning the hard way…
The solution was simple: instead of patching django.core.mail.send_mail I just patched the version that had already been imported in my views.py – myapp.views.send_mail. In other words:
with patch('myapp.views.send_mail') as mocked_send_mail:
...