1script - 5:38 pm on Aug 13, 2013 (gmt 0)
I am using a 728x90 which shows up ok on my samsung.
Try "responsive ad unit" - it will scale from 320x50 to 728x90, depending on viewport size. Your 728x90 may look OK for *some* ads but too small for others - you just never know, I assume you don't just sit all day refreshing you page to see what comes up. So, chances are, you're just not seeing the ads that look awful downsized by the phone to 320px wide from their original 728px.