Subject: How does the rate of catalase activity change with time?

I am a yr 13 A level student and i am currently doing biology coursework, 
investigating catalase extracted from potatoes.  I initially picked 100 
Red desiree potatoes from a local farm & measured the rate of catalase 
activity of 20 of the potatoes and then after 3 days i measured the rate 
of another set of 20 then again after 6 days, 9 days, 12 days, 20 days and 
21 days catalase activity went down, however by a very small amount, why?

Re: How does the rate of catalase activity change with time?