Earlier, my Synology NAS server DS213+ went to system hibernation mode (blue diode blinking) without problems. Suddenly, it went only to disk hibernation mode (green led blinking) and nearly never to system hibernation mode. I have tried quite a lot of things to fix it but without success. Now I think I know the problem – it is the DDNS service. Another service which may prevent the server from hibernation is QuickConnect service. Try to disable these two and you will see if this helps. Of course, I did not not mention other well-known issues about hibernation mode which you have to check yourself. Good Luck!
UPDATE: If you set WOL (Wake On LAN) in the Synology Asistant application, your PC will wake up the NAS from the hibernation and sleep mode, even if you do not hit the wake-up button. Be carefull, another tricky thing.