>I had an index like this on a table:
>
>

>   CREATE INDEX [IndexCombo1] ON [Reply] ([NoMember]) INCLUDE ([NoThread]) ON [Reply]
>

>
>For some kind of reason, this was not longer optimized. I had to delete the index and recreate it. What could have caused this situation?

The other suggestion I have for you is to ask this question here
http://social.msdn.microsoft.com/Forums/en-US/transactsql/threads

You'll have much better chance of getting an expert's advice.