Flat Carling Extra Cold


I am having problems with my Carling extra cold, most of the time it is fine but when we get busy it runs flat on about half the pints. I have called cellar services and they say it is fine but they are only there when we are quiet or closed, I have replaced the glasses ( renovate every 4 weeks)and the glass wash machine, changed the detergent and rinse aid. I run co2 with one bottle for all the lagers and the others are fine.

I have kept a diary of the customers and staff that have the issues and have been able to rule out peanuts and perfumes. it is always fresh clean glasses that i have the problem with. any ideas?