If loudwolf sell it as pcb etching solution & you mix it at the right concentration it should work, something must be stopping it working for some people,
It could be your water that's no good especially if its got iron in it,
I use Ferric Chloride in distilled water & it never fails to work,
Ammonium persulfate does work on clean copper when its mixed in distilled water,
in tap water the results will vary depending on what else is in the water,
good luck..