WordNet sense 1 (producing no result or effect):

futile, otiose, unavailing

WordNet sense 2 (having no beneficial use or incapable of functioning usefully):

useless

WordNet sense 3 (lacking in power or forcefulness):

ineffective, unable

WordNet sense 4 (not producing desired results):

unproductive

WordNet sense 5 (not producing an intended effect):

uneffective