The little guys that your character send out do the digging, if they can't get to where you place the shovel marker they will never dig. Start digging close to your character.
Display idea: Right now when you can afford 0 of something, the button is greyed, when you can afford 1 or more it lights up. Make it so that when you can afford whatever multiplier is selected (assuming you'll implement more than x10) the background turns gold.
 "Dungeon Defender"){kind=icon}
 "WhiteboardWar: ChopRaider"){kind=icon}
 "AdVenture Capitalist"){kind=icon}