Search in El Capitan

Search in Finder, search via iCloud Drive and search via Spotlight will not work properly - entire files with the search term are omitted. For example, if I search for a particular word in documents present on the MacBook Pro's drive, or an external drive, not all applicable results are shown. For MS Word, it appears to omit older documents (if I change the document to my current version of Word, it will get picked up). For saved web pages, I can't figure out what it's doing - sometimes it misses old files and at other times, new ones.