The most common cause for this is an invisible object covering the buttons. Many objects could stand between you and the table without your realizing it, such as invisible HUDs, certain shields, and alphas from nearby objects — most often plant foliage.

To check if this is the case for you, open the View menu on your viewer and turn on the “Highlight Transparent” feature. Alternatively, you can press Ctrl+Alt+T on your keyboard. This will cause any invisible objects to appear red and foggy. Make sure there are no red prims covering the table that you are accidentally clicking on, or invisible HUDs obstructing your cursor. Use the View menu or press Ctrl+Alt+T again to turn off the highlighting.

If no invisible objects seem to be covering the buttons, please consult this FAQ entry.