pens, pens, pens

Why is it that everytime I go to buy more of a specific kind of pen, I can never find it again? It seems to me that if I buy rollerball ink pens, I can never find them again at the same store in the same area I found them the last time! I never have problems finding those papermate black or blue pens, but those .5 or .7 mm tip pens with fluid ink give me problems. Call me crazy, but I don't think pen shopping should be this difficult.