SQL 2005 identity field is not getting set automatically during insert

Hello,
I have a SQL2005 back end /Access 2003 front end application. This was converted from SQL2000 a few weeks back and worked fine till two days ago. Now the main table's identity column is not working as I expected. From the server, I opened the table and added a row. I saw 'Null' for the identity value with a message indicating some problem. This message indicated that I should run 'execute sql'. When I did that, the identity field got incremented. This is a production database. Identity is set to true, increment set to 1. Seed was set to 0 way back when and hasn't changed.

Any suggestions?

Thank you in advance,

BA.