Like whitespot it is not what you see on the fish that you are treating, it is the vegetative form in the water. You need to treat until there are no more showing on the fish and then some. The easiest way to spot velvet is with a torch at night when it is dark. If whitespot looks like salt on the fish, velvet looks like pepper. There can be more resistant forms of velvet and if things are not improving then raising the temperature may encourage the resistant form on the fish to release the weee darlins the acriflavine will get at.