The green line defines the frames that After Effects has stored in RAM for preview purposes. This is called a RAM Preview. RAM Previews are limited by the amount of available RAM in your system and its allocation within After Effects.
If you can't generate enough RAM Preview, it's because you don't have enough RAM to store it. Some solutions include:
Reduce the resolution of your preview to half or more.
Render a preview to a file for previewing, either through the Render Queue or Adobe Media Encoder.
Check your RAM allocation preferences within After Effects to ensure you are utilising your hardware to capacity.
Add more RAM to your computer.