Composite shot goes black at original ending when reducing speed despite extending duration
So i've got Hitfilm Express.
So say I've got a 10 sec clip, I'm reducing speed by 0.5 so that it becomes a 20 sec clip. I change the settings of the composite shot to become a 20 second clip, and then extend the bar of the track to 20 seconds. Regardless, the clip goes all black at 10 seconds, even if its just half way through the actual video.
This seems awfully odd, and how do I fix this?