What kind of animation are you using to bring in the line of text? A motion
path? If so, then you must remember that a motion path is not an entrance
animation. So to fix it, you have to add an Appear entrance animation, put
it immediately above (before) the motion path in the animation timeline, and
set the motion path to begin playing "with previous."

I'm guessing you are using Motion paths?
Delete the effect you are having now.
Next, click Add effect > Entrance > Fly In.
Then, set the direction to "From Bottom Left".

Alternately, to get closer to what you are having (that is, to have the text
positions on the screen when it starts moving), click Add effect > Entrance >
Appear.
Re-order the effect so that Appear is before the motion path. Set the Start
of the motion path to "With Previous".

Feel free to send me a copy at pptheaven[at]gmail[dot]com if problem still
persists.