If you take a bunch of pictures in one spot, but only some of them get geo-tagged, you can copy and paste the location from one photo to another. Simply select the photo and use Edit, Copy or Command+C like you were copying the whole photo. Then select the other photo and select Edit, Paste Location to paste just the location of the first photo into the second. You can continue to paste the location to more photos.
Mac Video Tutorials and Tips for Apple Users
