What exact version of AE are you using? 11.0.0? 11.0.1? 11.0.2?
Is your playhead at the end of the comp?
What's in your comp? (Effects, number of layers, format and codec of footage, etc.)
How are you trying to invoke the RAM preview?
What are your computer's specs?
And all the rest of these pieces of information would be helpful too.