I find this is better achieved by creating the title in Photoshop and keyframing the vertical Position to simulate a roll.
What is the dimension size of the credits title that you created? Without being able to see your motion settings in the Effects Controls Panel, it looks as if the title is either not the same size as your sequence, or that maybe you repositioned or resized the text in the Effects Panel. If that doesn't solve your problem, can you include a screen shot of the size of your title, and another screenshot of your Effects Controls Panel?
Did Brandon's advice help?