It seems to work fine for me. The problem might be that your computer/monitor resolution won't allow it. You can try raising your resolution on your desktop to match whatever resolution is required on onigiri and try full screen mode again.
For example.
1. Go to control panel>Appearance & Personalization > Display > Adjust Resolution.
2. Change Resolution to 1280x768 / 1280x800 / 1440x900 / 1920x1080
3. Go back to onigiri & choose the resolution you changed to and try ticking full screen.
If that doesn't work, I have no idea whether or not you will ever be able to play full screen.
On a side note: Full screen for onigiri is utterly and completely useless. There's so many bugs with full screen, when you alt tab sometimes, that game will freeze or disconnect. Other times it'll take CENTURIES to alt tab back into, and by some chance if you were to alt tab while loading.... Rest in Peace.