When I search for files containing a specific string using the Windows
Search feature, often I find that Windows doesn't retrieve the correct
search results.

For e.g. today I invoked the Search feature to search for all files
(using *.* in the first textbox) in a folder named C:\Inetpub\wwwroot
\ASPX that contain the string '.Math' (without the quotes). I am more
than 100% sure that there are some files in the above-mentioned folder
which have the text '.Math' but the Search says "Search is complete.
There are no results to display".

Why isn't Search retrieving the correct result?

Please note that this isn't the first time I am encountering this
anomaly. I have come across similar situations on numerous occasions
when I am sure that a particular string exists in files in a folder
but Search doesn't retrieve any results.