Getting the selected value always false of the Check box List

I am facing an issue i.e, getting the selected value always false of the Check box List.
I am having two checkbox list in the page and on button click,submitting the data.
The  Selected values for the first check box list is coming right,but for the 
second checkbox list the  getting selected=false,even on selecting.
Both the Checkbox list and button are in the single update panel.