We can't know. You have not provided any system information or even told us what version of AE you use nor any info about your compositions and what not.
Mylenium's right. We need more info:
And a nice, long visit here wouldn't hurt, either:
If you're using CC 2014 and your OS is Yosemite, there is a known bug where a RAM preview won't show the first frames - HOWEVER, if you let the RAM preview loop back around, it'll play through just fine.
See the known issues section here: After Effects good to go with Mac OSX v10.10 (Yosemite)
If that's not the issue, more info as the others have suggested would be good.