I just installed after effect CC 2015 today, then I try to use it. I realized that RAM preview options was missing, so when i want to preview the video i use spacebar (preview), but the preview doesn't play the entire sequence, it just 2-3 seconds, and sometimes the dialogue box appear, it says it needs 2 or more frames to playback.
How to fix it?
Those are simply bugs. Beyond that nobody can tell you much. You have not provided any useful info about your system and your project.
It's Win 10 single language 64bit
Core i5 @ 3.40GHz (4 CPUs)
And about my project, i just add warp stabilizer VFX into my comp and when I press spacebar it didn't show the entire comp