I'm not sure if this is a problem specific to my game, but....if I try adding feathers to the box in Villa Auditore, or Codex Pages to the wall, or Assassin's Seals for Altair's Armour, the game won't add them if I have the trainer activated.
It even bug's the game if you've had the cheat activated recently, then disable it, so the best thing to do is go to Monterrigoni, make sure the game saved, then quit, disable the trainer completely, and re-load the game before you add them.
Also...regarding the seals, if you DO try adding them with the trainer activated, the game will 'place' them in position, and unlock the gate BUT...the gate will not actually open for you to recieve the armour, the statues will not let you put the seals on again and so it becomes impossible to get the armour of altair.....if anyone knows how to fix this, I'd GREATLY appreciate it, I really don't want to lose a save I've collected every seal, every feather, every treasure chest and every Glyph so far.