I'm having the same problem. I'm on these forums to determine if anyone/everyone else has reverted back to CC 2014 because of issues like this.
Whilst I don't like to hear of others having trouble with their software, I'm glad that i'm not the only user having this exact issue.
There is an update coming out that fixes a lot of previewing issues in CC 2015. According to Adobe, if all goes well, it'll be out in less than a week.