Any reasons that prevent Apple Remote Desktop to wake sleeping Macs?
"Wake for network access" refers to the system receiving a special "Wake On Media" or "Wake On LAN" packet from the local ethernet network.

Simply trying to connect with a network application will not wake it up.  That's because while the computer is sleeping, it cannot process any network traffic.  WOM works at the hardware level of the ethernet card: the card stays powered on and when it sees an ethernet packet with its MAC address and a certain byte pattern, it wakes up the computer.

Because WOM is based on ethernet frames, it generally cannot not work by itself outside of your local LAN.  Some routers provide utilities which let you remotely instruct the router to generate a WOM packet, but not typically consumer grade routers.