Normally the blenders are very reliable and quite accurate. You may want to inquire as to availability of a
gas analyzer which can test the blend ratios applied to these products. Possibly your stout distributors have access to this device.
Other areas to check would be line cleaning practices, brewer's carbonation spec. tolerances, gas leaks, gas vendor quality, product age, and others. Keep us posted as to your findings.
Scott Zuhse, Instructor
Micro Matic Dispense Institute