I have an iphone repair store too.
I am having the same problem.
I have two different aftermarket glass supplier.
70% glass works on replacement. Some of them doesn't work. Then we use other suppliers glass or same suppliers glass. It works. We use Non working glass on another customer. It works most of the time.
It looks like software problem.
Once, none of the glasses didn't work on one of the customers iphone 4. After 4 replacement last try also didn't work. I was hopeless. I put my finger on it many times. No way. I put on my ear no way. I kept on my ear like 20 second. After 20 seconds it shot off. After that it started to working fine all the time.
Software. Software. Software.
What happens (I guess) when we replace the glass software trying to look for background reflections, brightness. Doing some statistics etc. etc.
Software. Software. Software.
Now replacing iphone 4 glass is nightmare nightmare nightmare.