f5 update check manually working but not automatic

We have LTM 1600 in pair (A and B)

 

System ›› Software Management : Update Check

 

I have checked firewall/DNS etc.. everything working fine. I did telnet (443 port) and it works.

 

A - Automatically working OK

 

B - Automatically not working. But if i do manual check, it works!.

 

How do i find out what and where is wrong?