The other point - and forgive me if this seems blindingly obvious to you, but it appears to be anything but to a large section of the Argentine press pack - is that Argentina don't play like Barcelona. When Messi gets the ball for Barcelona in the midfield area, he's typically got plenty of options: Daniel Alves, Zlatan Ibrahimovic, Andres Iniesta and Pedro will all be making runs ahead of him, with Xavi likely to offer a short passing option. Messi has three things in this situation: space, team-mates giving him quick (and quickly changing) options, and his own ability. Barca's high-tempo, high-pressing game is perfectly suited to getting the best out of him.
By contrast, Argentina play at a relatively sedate pace. They like to retain possession and work an opening, but there the similarity to Messi's club ends. In his country's shirt, if Messi picks the ball up in that same midfield position, he'll have support behind him from Javier Mascherano, Juan Sebastian Veron et al, and might have, say, Angel Di Maria and Gonzalo Higuain ahead of him. The paucity of options high up the pitch is startling - especially for a side with as many frankly brilliant attacking players as Argentina have.
In this system, it's difficult for Messi to flourish, because his game hinges on either using the yard of space a team-mate can buy to do the unimaginable, or choosing his darting skill and passes - and he's improved in that respect immeasurably in the last two years - to help work the ball up the pitch. That passing and linking is still there when Messi plays for Argentina. It's the creative spark, born of options from team-mates, that's missing.
In the few times that I have seen them, it also seemed like Messi got less touches in general with Argentina than he does with Barça.