[Tom Turner]

Just came across this old post, as I was having the same issue – RAM previews (and the green line) would disappear the second I did anything – and I found the source of the problem…

Eventually realised that AE was RAM previewing at full quality, even though I had it set at ‘Quarter’ in the Composition window. However, in the Preview pane, Resolution was set at ‘Full’. So when I rendered, it would render at full quality, but as soon as I did anything, it would default back to the project-wide setting, and the previews would appear to vanish. If I change the Composition window setting to Full, the preview reappear, and if I set the Preview pane setting to ‘Auto’, all is well ☺

Hope that helps anyone else who experiences this hiccup!