obviously, murray can hit a dtl fh but it's true that the shot isn't as natural for him as for a federer or djokovic. murray hits his forehand with a closed wrist most of the time, his natural contact point comes with the racket lined up with his forearm. great for cc fh, not so great when you have to lay the wrist back to take it dtl (in an open stance, at least) or inside out. usually you see this type of stroke on guys with eastern/continental forehands due to the natural wrist position, but for some reason it's how murray developed his fh.
of course it's hardly a 'weakness', he's a solid player on all counts, but this is a slight technical issue with his mechanics. like i said, he can do it, i've seen him rip more than a few inside-out fh's, but it's definitely not a shot that 'flows' for him like it does for roger, novak, nadal.