Hi Sudheer,
I had faced similar issue earlier and as mentioned above by experts those can be the resons.
Mostly, in Delivery if your Output Type has any issue, system will give Update Error which you can see either via SM13 or SBWP in your SAP Inbox.
You will get exact error what it is and refer it to your ABAPer.
If your Number Range is finished is similar type of another example for error.
In General, while any transaction updates, system generates Number and shows it to you, but updates your Document number in Data base table after finishing all activities like triggering Outputs and generating Spools.
If it gets error during that processing, You will not be able to see Delivery document in VL03N it self or process it further.
Regards,
MJ.