It depends very much on what other settings you have. E.g. inter-letter stretch and words with ‘missing letters’ are incompatible with matching-based hyphenation. There’s also an issue with there being a limit to the number of words that TeX can hyphenate based on exact matches (which is the method TeX uses from a wordlist file). I think that syllable-matching hyphenation works differently.
Is the issue that hyphenation is not happening at all, or is it in some places, but not in other places where you’d expect it to?
A ‘minimum broken example’ as an archive would probably help us work out what the exact problem is. Whether it’s easily fixed is a different issue…