I've had it happen where the system would not sleep but never where the screen saver wouldn't start at the right time.
Try this first. Logout and then log back in but when you enter the password hold the shift key down and then click login. This will start the system with nothing running. See if it goes to sleep on the timer.
If that doesn't work, try starting in safe boot where you restart the machine and hold the shift key down at startup. It takes a while to boot however, this cleans things up. You'll get the login prompt. Now see if it will go to sleep on the timer. If it does, there's something running in background that's interfering with the process.
Finally, try one other thing. Create a test user account (you can delete this later). Login to this account and set the timers for screen and system sleep and see if it sleeps. If it does, there's something in the user account that's affecting sleep. If it doesn't but it slept in safe boot there's something in the system startup that's interfering. That's all I can think of.
If all this fails and you were to call Apple for support I suspect at this point they would tell you to do an archive and install.
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