Debug point not fire in visual webpart update panel

Hi Eveyone, 

I am facing an strange issue, I have written a visual web part  with update panel. The code is working fine but when I click on the link button the debug code will not hit :S 

When i place the same code out of update panel the debug cursor fire and it's pointing me to the relevant function. 

The problem is only arriving with the link button and i did not get an clue why the system behave like that. 

If some one have an idea please response my query.