That game has done that to me, restarting the game and steam completely has fixed this issue for me, apart from being frustrated.
When you inject and it says trainer activated, sometimes its not activated and takes awhile so just play the game and leave it, it will say trainer activated again, usually on the third the options 1,2,3 work, pressing 4 you wil hear trainer activated again. then it will work.
Oh... no wonder... So that's what happened... This is just like forever mutating virus in your body that you have additional injection after it mutated(upgrade of toughen itself). LOL
And Frosty... Don't be offend but your avatar reminds me of the game Amnesia and some shopping mall female manikins that has its mouth open... it's sick. Just wanna inform you. Sorry...
[Edited by Ryuuichi, 3/17/2014 8:53:57 AM]
[Edited by Ryuuichi, 3/17/2014 8:54:20 AM]