Immobilised Enzymes: A2

Hi,
I did an experiment in school last week investigating the effect of temperature on both free and immobilized lipase. We used temperatures of 40 and 80 degrees C. At 40, both were active but at 80 only the immobilized lipase was active. However, its activity was greatly reduced to that at 40. I understand that the immobilisation process makes the enzymes more stable but why is this and why, if it is more stable, is the activity reduced at all at a higher temperature? In other words, what is the explanation for the decrease in activity in immobilised lipase between 40 and 80 degrees C when with free lipase the enzyme is simply denatured and activity is completely halted?
Any ideas would be greatly appreciated!! Thanks