If you run any AE version earlier than CS6, you'll need to transcode it to a different media container & codec.
Assuming you have something earlier than CS6, this may be a factor but what version ARE you running?
Are you saying that it shows fine except during a ram preview?
There are other considerations as well, such as PC processing power, clearing cache, reducing to half resolution, etc.
Usually those symptoms are not no preview but sluggish or short lengths will show. Can you describe a bit more?