I can see the free busy times of newly created test exchange users of my organization, but not the existing users who have been in the exchange system for quite some time.

In debug mode this comes, but his FB times are not shown on zimbra (old exchange account)
<a:fbdata>0000000000000000000002200000000000000000 00000000</a:fbdata>

Following are the new test exchange accounts showing FB times properly to zimbra
<a:fbdata>0000000000000000000000000202000200220000 00000000</a:fbdata>
<a:fbdata>0000000000000000000000000000000000000000 20000000</a:fbdata>

Could someone help me please?

Thank you.