fail vs fall flat

fail and fall flat both are verbs.

Word	Noun	Adjective	Verb	Adverb
fail	No	No	Yes	No
fall flat	No	No	Yes	No

As verbs, fall flat is a hyponym of fail; that is, fall flat is a word with a more specific, narrower meaning than fail:

fail (verb)	fall flat (verb)
stop operating or functioning	fail utterly; collapse
deteriorate
prove insufficient
disappoint, prove undependable to; abandon, forsake
become bankrupt or insolvent; fail financially and close
be unsuccessful
fail to get a passing grade
fail to do something; leave something undone
judge unacceptable
be unable
fall short in what is expected