This seems to be the trick to defending in PES 2012, although by no means the remedy.
My advice with regards to defending in PES 2012 is to largely forget about the RT 'hold up' function as it has clearly not been properly implemented and attempts to use it against attacking play that moves far too quickly will most likely just leave your defense exposed. A better way to think of the RT/R2 function is as the 'leave gaps function'.
It is a bit sad to say that in PES 2012, you want to let the AI do as much of your defending for you as possible. Use the :square: to call for a team mate to pressure and manouvre the player you are controlling to block any gaps that are opening up wihich your AI controlled defenders aren't dealing with. Worthy to note, is that constantly holding down the RB/R1 button to run when defending is a cardinal sin, only sprint if there is a clear need to sprint.
If your teammate is pressuring well, then it way be worthwhile to snappily hit the LB/L1 in order to change to the teammate in order to make a quick snap tackle, but most of the time you will either give away a freekick or your player will happlessly attack an area of space behind the opposition player.
The bottom line is that defending is fucking awful in PES 2012 because Konami are in the middle of changing the system and have not got enough of it finished in time for the release of this game.
What version of the 'full game' do you have and what region did you get from? i.e. is the game 1.00, 1.01, etc?
I have pre-ordered the UK release for the PC and in the meantime, am enjoying my 'unofficial trial full version', which has been patched to 1.01. Whilst their are slight improvements, it isn't like night and day and the defending aspect in the game is still very poorly calibrated.
I can see quite a few online rage quits with this crap as Messi et al glide through about 8 defenders with as many yards as they slip and slide around all over the pace, none of them dare showing the audacity to actually put a foot into the ball (without fouling....defenders in PES are great at giving away fouls)