Display acceleration gets disabled usually when the GPU is not getting utilized. As @Mylenium mentioned, it is difficult to know the cause of any problem or provide a solution without getting the complete details. I have a few questions:
Which version of After Effects are you using?
What are your OS details(version, RAM, GPU, etc)?
Have you updated the GPU drivers to the latest version?
No magic switches. It just requires your system to be up to speed. Start by updating your graphics drivers. Of course even then it can still be disabled if you use effects and features that don't support acceleration features. Impossible to know. You have not provided any information.