#1 Turn off all AV products (I have no idea why people have been saying that AV products interfere with trainers, my previous 3 AVs and my current one has never interfered with any trainers before 0.o)
#2 Check game version, check trainer version. Both must be the same for them to work. Usually trainers modify certain memory locations in game. After a new patch these locations might have been modified and cause the trainer to not be able to recognise them/modify them. A new trainer will most probably be released with every new patch for the newer and more popular games.
#3 If you are using Vista, run BOTH the GAME and the TRAINER as administrator. And i don't mean logging in with an administrator account to run them. I mean, right-click on both the trainer and game shortcuts/exes, and clicking "Run As Administrator".
Vista doesn't give a damn if you are using an administrator account. All programs that are run are still treated as running in a standard account unless told otherwise.
Well for example I have downloaded an older trainer,the +6 one and then I have downloaded the 1.2 trainer and I even patched the game with 1.02(same as 1.2).After that I tried and tried and nothing good has shown up.