Trying to record something window to full screen. Help!!!!!

I'm trying to record the sims 3 game. So, when i begin recording, the game is (obviously) not full screen, so i can start the recording going and check that everything is working. When i make it full screen (by pressing return and control, the only way to my knowledge) it gets all messed up. I can only see the game in one corner of the screen where is was before when it was a window, not full screen. The rest is just colored lines. Please help?! If you need more information tell me please 😕