Actually the 'some' is ALL YouTube videos, but there is already an open post on this topic (which hasn't so far suggested a solution to the problem with my installation) and I'm told it's bad practice to hitch a ride on somebody else's post.
I've just installed SMPlayer (version 0.8.6) using (I think) the package manager on my Linux distribution (Mint 17 XFCE).
SMTube is version 1.8, installed using a ppa I think
MPlayer is version 1.1 according to SMPlayer 'about', but according to my Package Manager it is Version 2:1.1+dfsg1-0ubuntu3
The problem is that SMTube starts OK, I can double click on any video but SMPlayer does nothing aside from telling me that Subtitles are on.
What should I be checking/changing in order to get the results I expect?