Glass not level with chrome ring

I've taken apart 4 iPhones. Only one of them was to replace broken glass. All of them have the issue upon reassembly that the top of the glass (near the earpiece speaker) does not sit as flush with the chrome ring as the bottom of the glass does. Is there a trick to making it sit level all around the glass? I tried pulling the upper glass assembly back out and making sure the tabs on the upper glass assembly matched and sat in the notches in the upper part of the chrome bezel. The glass still isn't perfectly flush like an untouched brand new iPhone 3G. Any ideas? Thanks so much! Your website is great btw!

Will