Hello,
the file name probably contains whitespaces and you search for it by including the whitespaces in your query. But by default, Alfresco treats whitespaces as term delimiters and ORs the terms, e.g. a search for <blockcode>my important file.pdf</blockcode> will search for <blockcode>my OR important OR file.pdf</blockcode>.
If you want to search for the full name please make sure you enclose the entire name with quotation marks, e.g. <blockcode>"my important file.pdf"</blockcode>
Regards
Axel