Wiktionary
damp squib
n. 1 A firework that fails to go off, due to wetting. 2 (context idiomatic by extension English) Anything that doesn’t work properly, or fails to come up to expectations.
Usage examples of "damp squib".
You're the man who prepared the charges so, of course, we'll all blame you if the explosion turns out to be a damp squib or, alternatively, the palace falls down, so perhaps it's only fitting that you press the button also.