I''m not sure if it's related or not... but I had a very similar
problem with "following" users for non-list based items. In fact, even
when I would add new users synchronously, I had to put a sleep in
there of a few seconds. If I didn't sleep between follow calls, the
account would reflect that I had been following the users, but I would
never receive their status in my stream. (For example, the account on
the website would say I was following 10 people, but I'd only ever get
the status for 2)
If I put a sleep of a few seconds, everything worked perfectly. So, it
seems like the list-based following might suffer from the same issue
as the non-list based following.
I know it doesn't help solve your issue, but maybe others have noticed
a similar problem and twitter can look into it.