dopesheet

When drawing with grease pencil, new keyframes are inserted at the current frame. Because the main time-line view will not show grease pencil keyframes by default, it is easy to lose track of which frames have grease pencil animation.
The workaround is to switch to the dope-sheet view and enable grease pencil mode, so that you can see your grease pencil keyframes in the dope-sheet.