When selecting large areas with the magic wand it’s common to leave a few pixels here and there throughout the areas unselected. Changing the Magic Wand color similarity tolerance is the first step, but it doesn’t always do the job.
Sometimes easier and more effective is dropping into Quick Mask mode after using the Magic Wand tool. If, in the ruby overlay, you still can’t tell if pixels are selected, unselected, or partially selected, use the Channels palette. On the Channels palette, turn off all channels except the Quick Mask channel. Your selection will now be rendered in high-contrast–mostly in black and white. Any holes in your selection will be as obvious as stars on a moonless Midwest night.