Searchlight fails to find files on my Mac

You need to accompany your search with operators that can steer your search results.

kind:excel AND name:cost

will find any Excel .xls/.xlsx document whose actual filename is, or contains, the word cost. On the otherhand,

cost Estimates.xls

will search all of your selected Spotlight Search Results categories for a match, including internal document text, and the result count could be staggering. When you have a filename, or a text string punctuated by space, you want to enclose the string in double quotes, so both parts are not searched independently:

name:"cost Estimates".xls

See also: Narrow your search results on a Mac

If you want to limit the search to a particular category, select the search token from the list that pops up when you type in the term.

It will attempt to show you all the possible limiting categories from which that term will match. You want, Name matches

Do you mean, "Spotlight?"