What version of AE are you using (down to the last decimal)?
How are you previewing it?
What format and codec is the footage involved?
What are your system specs?
It's After FX CC, version 188.8.131.52
Either I play it pressing 0, Space or . on num pad, it won't work
Format is MOV, while codec is H.264
System specs: AMD 995; RADEON 7770
And yes, is this not working 'cause of low memory on the disc?
That is an incomplete list of system specs. All you told us is an incomplete CPU number and your GPU (and AE doesn't use the GPU for much at all).
What operating system are you using? Mac? Windows? What version?
How much RAM?
What kind of hard drive(s) are in your system? How much space is on them?
Also (and this is probably the issue), you are way behind on updates.
If you want to stick with CC, you should consider updating to 184.108.40.206 - there are a lot of bug fixes there.
You might also consider updating to CC 2014 (the latest version of that is 220.127.116.11).
You also might consider updating to CC 2015 (the latest version of that is 13.5.1).
If you update to CC 2015, make sure you go into the advanced settings of the Creative Cloud app and set it so that it DOESN'T remove your old versions of AE. See this for more info: http://adobe.ly/1MQ48cW
Also, not only are you missing out on bug fixes, but you're missing out on a number of new features too!