I am running a server and currently it has only:
Essentials, EssentialsAntibuild, EssentialsChat, Autosaveworld, PreciousStones, EssentialsGroupManager, EssentialsGroupBridge, EssentialsProtect, EssentialsSpawn, Worldedit, and Worldguard.
I type in my server /mangaddp default preciousstones.benefit.* which according to the wiki is correct, and still players cant use launch pad or sky cannon. They can use practically everything but those two...

Are the players placing the launch pad or sky cannon block?
Or are the blocks ones that you've placed for them?
I believe, if you placed the blocks, you need to '/ps allow' them into the fields in order for them to work.

If the issue is occurring when they place the block, does it say "launch pad field placed" when they place it?

The other players other than the owner wont be able to use them unless the owner allows them with /ps allow. Could this be your problem?