"Fell flat" is a common idiom that is often used to express disappointment or failure. In order to convey the same sentiment in different words, you can use synonyms such as "flopped," "bombed," "failed to impress," "met with a lacklustre response," "didn't generate interest," "didn't go over well," "received a lukewarm reaction," "missed the mark," or "was a letdown." Each of these phrases can be used in different contexts, depending on the situation. Remember that using synonyms for "fell flat" can help you convey your disappointment in a more varied and interesting way.