SPGridview refresh behaviour

In SharePoint Page , 2 webparts are added

First one is custom webpart  - SPGriView  added to updatepanel

Second one custom webpart with button which refresh  SPGridview in first webpart

This button  also changes the values of one of the column in SPgridview

for the first time  when button is clicked it shows  that SPgridview will not be changed but for second time  will show the changed value which supposed to be visible on first click and so on

any ideas , why SPgridview doesnt show updated value for first click and for further clicks it shows the  older values?