Adobe Illustrator defaults to hiding the grid behind your artwork. If you are working zoomed in a lot and need more precise snapping here is how you enable the grid to show above the your artwork.
Select Guides and Grids in Adobe Illustrator's Preferences.
Deselect the Grids in Back option and click OK.
The keyboard shortcut for showing and hiding the grid in illustrator is Command⌘ + ' (key to the right of the semicolon).