That seems to be a problem with youtube in general. I see that behavior all the time. Several local friend of mine also have these seeking problems, too, so it may depend on the particular cache servers you end up seeing.

I think the problems started when they changed to the "adaptive" rate stuff; the players don't don't download the entire video file anymore, and instead they buffer in segments. Not only does this remove the ability to pre-buffer the entire video, the client seems to get out of sync with the server. The client ends up waiting forever, but the server isn't sending anything.

Fortunately, I was able to switch many of my friends over to youtbe-dl, and that fixed everything. Unfortunately, the article mentions the idiotic "encrypted medfia extensions" a one of the reasons for the changes to youtube; I wonder what additional useless hoops youtube-dl will end up having to have to jump through.