Problem with SUS update with clients..

Hi,I have a network with over 400 computers..

I have a local SUS server and my group policy is configured for
automatic schedule and installation everyday.

Today i noticed that about half of my clients didnt make any update
till 1 year.
Actually i couldnt find any reasonable reason about it?
Can it be because of users that use that computersmight be restricted
user? Or what can be the other reasons?
I am a local admin in my computer and my computer gets all updates from
local SUS server immediately..

I will appreciate with any ideas that might cause it...

Tnx..