Is it possible that you are failing to remove some small object that you can't see? Are the punctures in the same place each time? reason I ask is that failing to remove some tiny thing embedded in the rear tyre that is just every so slightly poking through, is a common cause of repeated punctures in my experience. I have found that turning the rear tyre inside out and really examining it well helps considerably, I have even been known to resort to using my tongue (it is very sensitive and can feel thinks you can't easily see) on the inside of the tyre where I know the puncture has occured. I know it sounds mad, but it works and is not as unhygenic as it sounds (honest!).
Anyhow - it was a random thought for you....