It seems like participants doesn't look farther back than the most recent entries--i.e. it works on entries that are very recent but not on entries that are further back in the stream.
Tested this on all_feed and statuses_feed.
This is due to the indexing being done at sync-time, and feeds can't be re-sync'd older than a certain limit. Everything since the participants rollout a few weeks ago now should all be indexed properly, only feed data before that won't/can't be :/