When doing a Keyword, Global, or 'for text' search for part of a word, the expected results may not get fully returned. For example when searching for 'serv', partial of 'server', the search does not return all results like it would for 'server'.
This is the expected behaviour of the platform, as the text search engine only uses complete indexed words, or word stems. The third party (Porter) adopted word stemming procedure does not equalize to word truncation, such as to expect returning results implying a wildcard.
In order to search successfully with a partial word, make sure to add a wildcard such as * or %, correctly placed within the search term string. Plural words do not need a wildcard instead, the word 'server' has the same stem as for 'servers', so both results will be returned anyway.
If you are trying to get results for 'server', and using the word 'serv', your search string would need to be 'serv*' in order to retrieve the right results.
ServiceNow documentation pages on search wildcards: