Make sure you're injecting on start up. It has to inject right when the process is starting.
You can use my DLL injector, it has an option for Automatic Injection. https://guidedhacking.com/forum/index.php?topic=438.0
So, get everything set up before you start the game, check automatic injection, then start your game.
Also, if you're on Windows Vista/Windows 7, make sure you're running the injector's As Administrator. That definitely can cause problems.