When a RAM preview begins to play, the first several frames of the video do not play the first time through. Subsequent loops through the same RAM preview duration play all frames. This bug with Mac OS X 10.10 (Yosemite) affects all versions of After Effects.

Solution 1: update to After Effects CC 2015 (13.5.1)

Previewing a composition is changed significantly in After Effects CC 2015 (13.5.1), so frames are not skipped in during preview.

Solution 2: downgrade the operating system to Mac OS X 10.9, or earlier

Downgrade Mac OS X to 10.9, or earlier to avoid this issue.