All computers that are using HOMEGROUP have to all be on the same HOMEGROUP and have to be properly configured to share something before any of the computers will auto sleep properly.
Example, I had two computers on Win 8.1 Pro (which automatically starts a HOMEGROUP when installed) that would NOT connect using my old HOMEGROUP password. I created a new HOMEGROUP password and managed to get these two computers to share files. However, there was a THIRD computer on the network (with Win 8) who had the old HOMEGROUP password. I successfully changed it to the new HOMEGROUP by leaving and rejoining using the new password. Yet I could not see any files on this last computer. I had to UNSHARE with the HOMEGROUP all of the files and folders under users. Then I shared the one folder that I wanted to share. That worked and that also FINALLY let my computer sleep!
Two other things. The computers do NOT have to be booted up to cause this issue. You can easily tell if this is your problem by disabling your network adapter, setting sleep for one minute and waiting 5 minutes to see what happens.