The examples you'd given for missing threads all seem to involve updates that had occurred in less than an hour (sometimes only a few minutes) before you found them curiously missing from the latest post listings.
Some forum database updates run with a roughly hourly frequency. However, posting updates are immediately available once the updating user transaction is completed. There's no evidence of server breakdowns or bog-downs that would explain the incidents you've reported.
The only hypothesis that makes sense to me is that there's some sort of delay or update latency occurring in your long-distance connection to the forum server in North America. The most likely cause would be some sort of caching or buffering occurring along the bucket-brigade IP path leading to your (apparently wireless?) end point(s). Even if you had disabled caching on your end devices any buffering or defaulting to earlier content might occur as a result of overload or limitations with (e.g.) your service provider(s).
Most of your connections during October have been from nodes scattered around the UK belonging to a single telecom group, but you've also arrived via nodes operated by three other service providers. This may have something to do with it ...