Forum:Storage deck upgrades dont stick

OK, am playing Borderlands GOTY on my Macbook Pro. Good game, and am enjoying it, but have a question about the claptrap storage deck upgrades. I have 12 slots for my character, and when I get the claptrap awarded upgrade, for a short time I have see 13 of 12, but it never sticks.... Shortly afer that my slots go back to 12. Am I doing something wrong? Am I confused about SDU's? Help.