Diagnosis
The rubber foot on the bottom of the handle has probably worn down or gotten slick. That foot is what grips the smooth plastic of the charging base.
How to Fix It
First, clean both surfaces. Wipe the bottom of the handle's rubber foot and the top of the charging base with a cloth dampened with rubbing alcohol. Let them dry. Oils from your hands or lotion can make them slippery. If it's still sliding, you can try roughing up the rubber foot very lightly with fine-grit sandpaper to restore some grip.
If That Didn't Work
Check if the charger base is perfectly level on your counter. If it's on a slanted surface or a uneven grout line, it'll be tippy. Try moving it to a flat, stable spot.
Heads Up
Don't use anything sticky like tape or glue to hold it in place, as that will just gunk everything up and make proper charging difficult.