faire long feu

to fail to produce the intended effect

It describes something that starts out promising but comes to nothing, like a damp charge of gunpowder fizzling out instead of firing.

Son nouveau projet a fait long feu, personne n'en a plus jamais reparlé après le lancement.

His new project fell flat, nobody talked about it again after the launch.

Old muskets were loaded with gunpowder that sometimes turned damp, so instead of igniting with a proper bang it just fizzled weakly without launching the shot. That idea of a charge failing to do its job carried over to anything that comes to nothing.