There's not enough information to give you a good answer. What makes you think the GPU should be used, and what is letting you know that it's not being used?
The GPU is only used to render GPU-accelerated effects and other processes, such as scaling. After Effects uses the GPU minimally, but it's abilities are slowly increasing.
What is the content of your comp? Are you trying to send a comp from Ae to AME?
Please provide detailed information so we can better help you.