Thanks for the information. Could you provide the date code? Bottom of box, looks like it was done with a dot matrix printer.
I am sure Costco will happily replace the batteries. Kirkland Brand cells generally appear to be a Duracell product. House brands are made by the big manufacturers, but receive a house brand label and packaging. The Kirklands are not the latest Duracell alkaline design, but they give you 90+% of the energy of the latest design for less than half the price.
The mini mag is a high rate application, discharge in two hours. This should not be a problem for most AA cells, but high temperature storage (above 30 degC) can cause a problem, as well as use at low temepratures (below 0 degC).
If you want to know the real problem, check the cell voltage, and then try the same cells in a low rate application like a radio. If they don't work there, I would suspect high temperature storage somewhere in the cell's life.
Please keep me posted.
Spud