The first thing we can try is right click on the game window and select Auto Size screen. If this does not settle the issue, try this...
Find the resolution of your extended display, then on the preferences tab, click on Extended Display on the left hand side. On the dialog box that opens up, under the extended display enter the resolution and click OK. If your display is already on the extended display, move it to the main display and then back to the extended display. If the cells are still not showing up, right click on the game window and select Auto Size Screen then OK. This should resize all your cells. If you are using a layout you created, you may have to either change the cells manually (sizes) or, the easier way, would be to change the resolution of your extended display. If you choose to change the resolution, don't forget to change the resolution of that display through the Extended Display button on the Preferences tab.
Good Luck