You say you've tried fliers, discounts and coupons, but you don't say who received them. You need to get in front of all the employees in offices on other floors of your building and in adjacent buildings. 

Food is, of course, the ideal lure. I suggest dressing in the garb of a chef or baker and taking small boxes or baskets of muffins or bagels and menus bearing your coffee shop name to every front office. 

Ask to see the office managers. Tell each of them that the basket is a gift from your new restaurant and ask them to please put it in their lunchroom.  Also ask if you can provide menus for each department. If you're friendly, not pushy, you can win them over.