If your car is pre turbo MCSC then it could be few things. I not 100% about the later ones i think it same sort of set up
The gear change is done by two cables, the problem with this is there is a rubber gattor on the end of each cable, they split and water and dirt then get in to the cables making them stiff, spay wd40 on them while you move the gear stick in and out of gear see if it helps.
The cables do stretch but only if people have been hammering the gear Changes and would not think that's your problem
Also make shore where the ends connect to the selecter arm and go in to the gearbox are greased, which if you get some one to move the gear stick for you you will see moving if there not clean and regrease them
To look at cable ends, gators and selector cam you will have to pull out the air box assy It not a hard job
I would get you gear box oil changed anyway never hurts
I would recommend changing gearbox oil every two year or 30,000 miles or every second engine oil change. the oil deteriates with heat and moisture. Also if it never been changed it will still have the build swarf in it.
But like chris said if its under warranty take it back get them to fix it