If a member complains banners aren't showing on their site after adding the code it could be an anti advertising program is blocking them.

These programs that will recognise standard size images (such as 468 x 60 banners) as well as some words in the image filename and block their download and thus the image doesn't show on the site.

If you can see the banners on the site OK ask them if they are running any security or anti advertising programs such as McAfee, Symantec etc...