Magnesium Sulfate causes the parasites to die, but only if it ingested (see sticky for more info), not from contact in the tank water (except in extreme levels) - so the end result is the same as metro, only without the potential long term health issues. That, and it's dirt cheap and widely available. Of course if the fish is no longer eating then one needs to force feed, or switch to a medication that can be absorbed via the gills etc.
But as I said, way too many variables involved for myself or anyone else to determine why one treatment will work for one person/fish, and not another. I would need a crystal ball to sort out what others are doing in their tanks, with their fish. lol Josh's recommendation of Seachem Metroplex is probably your best bet, I have never seen metronidazole sold in liquid form. If you know a vet, ask them, it's a very common medication.