Switch Plate too Thick
I've used these simple extenders/spacers to move an outlet slightly forward. Basically, just unscrew the outlet's top and bottom screws from the box, bend and pull off as many of the green spacers (probably you'll only need 2 per screw) as you need, then place over the outlet screw holes and screw the outlet back in through the outlet metal tabs, through the green plastic spacers, back into the box. Note: If the gap is > 1/4", you must get one of these instead (by code).