When applying the Jetpack modifier to a Construct's Armory chestplate using leadstone jetpack or the creative jetpack, it works as expected - when you hold down space with the jetpack engine on, you fly upwards like you would when wearing the leadstone or creative jetpack. When using any other kind of jetpack - hardened, reinforced, resonate, or the flux-infused jetplate - the jetpack engine sound plays when you jump, but you just jump. You can't fly.