If you get some missing strokes or offset marks using the Slate, you can refresh it. This manipulation should solve your trouble and won't delete or alter your Slate in any way.
To refresh the Slate, press and hold at the same time the two top front buttons. The front LED will blink red to signal a successful refresh:
Make sure to keep the Pen, and the Ring if you are using it, at least 8 inches | 20 centimeters away from the Slate when you're refreshing it.
You can start drawing again immediately after refreshing.