What version of AE?
After Effects CC 2015 - Latest update avaiable until 1 March 2016.
Edit: I am the one who posted - I am at work and this login was automatic, that's why the name is different.
There is a known issue (mostly on El Capitan) where a cached playback will not play in real time. Your issue may be related to that one.
Alternative workarounds mentioned in that thread include docking all of your panels on one screen and making your Composition panel full screen so that it covers the timeline panel. The issue is mentioned in this blog post: After Effects and Character Animator ready for Mac OS X v10.11 (El Capitan).
The fix for the issue is targeted to be fixed in the next update to After Effects.
If your issue is unrelated, we may need to troubleshoot some more.