I've been playing the All The Mods 9 modpack.
On it's own, the instance of Iron Jetpacks inside there works fine, however;
When I add [Origins Forge], [Medieval Origins Revival] and [Pehkui] to the All The Mods 9 pack, the Iron Jetpacks mod stops working.

I've not pinpointed which of the 3 mods added causes the problems with Iron Jetpacks, but it's in there somewhere.

What happens ingame is that:
Any jetpack from the Iron Jetpacks mod (except the Creative one), will get an internal power capacity of 0/0. Which means the maximum amount of power the jetpack can have is 0, and hence it no longer works.