Search Feature not working properly

Hi Experts,

            I have TREX engine configured in my portals.

search function is working fine for some docs,but for recently posted items it is showing error as

<b>No matches found for search criteria</b>

But in actual,document exist.Earlier it was working.

Is there any problem with indexing?shall i do reindexing or any other settings which needs to be done.

Thanks in Advance

Amit kumar koyal