When I bring a clip into After Effects to slow it down, I always create a composition that is long enough to fit the final, slow-motion video clip (so, 50 seconds for a shot that was originally 25). After creating the comp, I drag my clip into the comp, then apply time warp. Here's where the problem starts. After Effects applies the time warp effect, but the clip length stays the same, with the whole second half of the clip chopped off. I try lengthening the clip, but it doesn't work. It's as if it forgot that the clip even had the second half of video there in the first place.
If I recall correctly, you'll have to precomp the footage in a longer comp to get the full length to show up. So if you are slowing down the footage by 4X, you'll have to make a comp that is 4 times longer and put the footage in the beginning. Then apply the effect to the precomped layer (not inside the comp).