A while back there was some issue with the wildcard expansion generating excessively long URLs that would be rejected by the web server. Jeff imposed an expansion limit as a workaround, which seems to limit the wildcard matches to the top 20 most popular applicable tags.
The problem is that the plus sign (+) is considered punctuation and that's dropped by the site search.
Even wrapping it in quotes (e.g. "google+") won't really help.
Alternatively, search through posts tagged google-plus which should narrow your field to the relevant posts.