let down vs letdown

let down is not a noun while letdown is a noun.

Word	Noun	Adjective	Verb	Adverb
let down	No	No	Yes	No
letdown	Yes	No	No	No

let down (noun)	letdown (noun)
	a feeling of dissatisfaction that results when your expectations are not realized

let down (verb)	letdown (verb)
fail to meet the hopes or expectations of
move something or somebody to a lower position