Ah yeah that can happen for a few reasons. If you leave the game open and the tab focused when you sleep your pc, it may not trigger offline earnings when you wake the PC. For html games, I'd recommend closing the tab.
Pressing the O key opens the offline earnings window where you can check if anything happened during the 12 hours. If nothing's there, it broke due to a random browser bug. The downloadable version is much more reliable in this way