I'm not sure if this will help you as I don't entirely understand when you want the computer to sleep and when not, but it may point you in the right direction.
On a recent pc i build/set up with windows 8, the machine would fail to sleep intermittently. Sometimes it would go into sleep mode properly after the define time interval in the power settings, other times it would not. IIRC all the usb devices were set to allow for reduced power/sleep mode.
After much research, I discovered that several services were preventing the system from sleeping at times.
Below are the services/programs I set the pc to ignore, and regardless of what their request is, to still put the system into sleep
\Device\HarddiskVolume2\Program Files\Windows Media Player\wmpnetwk.exe SYSTEM
To figure out what might be causing issues on your end, issue the command "powercfg /energy" from an elevated commandline. Do not touch the computer for a min. Examine the resuling energy-report.html, it should shed light on what's preventing the system from sleeping. You can also try attaching it here for review.