I too had a similar issue. I'm posting because this thread is one of the top results Google gives for Windows 10 sleep problems.
The solution that worked for two computers I had that were not sleeping was as follows:
- Run an admin command prompt
- powercfg -requests
- This command lists all processes or services currently running that are preventing the computer from sleeping. The list is helpfully broken down by categories. One of my affected computers had sleep blocked by the IncrediBuild component of Visual Studio, the other by a different TSR program.