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Check your /island settings and make sure the ore generator setting is allowed
If you attach to the roof it wont spawn above and i put it as, wall then air then spawner and it doesnt spawn outside so just make sure there is...
So after a few days of them being offline it should take the sign away and you cant get the stuff out.
It sells once every 10 sec. If you are not in the same chunk it sells a random amount but if your in the chunck it sells more. I think its worth it.