Yeah, Mercs2 is one buggy game.
I had a tough time getting the co-op trophies.
And several times, I would ask the chopper pilot to bring me something and it would vanish into thin air right in front of me.
Also, I am now in Act II on Veteran.
This game is not nearly as hard as CoD4 nor has the grenade-fest of WaW. You just take your time and go slow and you can win it.