I've had this exact same problem on my Dreadnaught acoustic, that doesn't have a cutaway. I can only barre that high up by shifting to "classical guitar" position. I thought I was crazy doing that, until I saw this tread :-)

It's worth noting that I CAN barre reasonably all the way up at the 12th fret on my electric stratocaster, with those lighter strings and cutaway. So the type of guitar and the strings matters here too, I think.