Tell us exactly what you're doing and exactly what the result is. Based on your vague description, we can only guess.
I'm going to guess that you've created a layer based on your footage item and then slowed down the playback with time remapping. The layer is still the same length, but because the footage in it is now playing back more slowly, that original length isn't enough to play the entire footage item. This is normal. Extend the layer to the necessary length to play all of the footage.
I'm also guessing that you're very new to After Effects. I recommend that you start here to learn After Effects: http://adobe.ly/bjBT3P