Tony, You didn t describe how you were monitoring the catalase reaction, so I m going to assume that you were judging the rate of the reaction by observing the formation of bubbles of oxygen gas.
Catalase decomposes hydrogen peroxide into water and oxygen.
Like any enzymatic reaction, the rate is described by a formula called the Michaelis-Menten equation. This number gives a measure of how well an enzyme binds its substrate. A good way to think about this is to remember that the enzyme is a catalyst, and is present in small concentrations. The concentration of the catalase is much less than that of the hydrogen peroxide. Once you have enough hydrogen peroxide present so that all of the enzyme molecules are rapidly binding hydrogen peroxide and decomposing it, adding more hydrogen peroxide will not make the reaction go any faster.