I had noticed I was having this issue too, and my screensaver never kicking in, and just going to sleep with it never kicking in.
I had gone in and checked to see if any of the settings had got changed or deleted in the system upgrade, but they were all still set at what I had them at prior... so, did as some attempted above also, and adjusted the times for it to kick in, and still nothing. Worked when I would hit 'Preview', but still not kick in at the interval time.
What I just did to get it to work, was just to simply go into System Preferences>Desktop & Screensaver, and just reselected the folder of images that I want to be used, in my case, a folder of images in my iPhoto. And it now works.
Apparently, it wasn't working, because, even though it was still selected to use that folder in iPhoto, something must have changed in the upgrades, which might of had something to to with the iPhoto update too, where it had to update my iPhoto library upon relaunch... and my screensaver settings were set to access that older directed to folder within Library, that was updated in the new release... and so, just repointing it to that folder now in iPhoto, got it to work.
At least, that is my figurin', and I'm stickin' to it! 😉