Unable to etch part # on Black Oxide

I received a batch of 3000 pieces of our wire wrapping sleeves from a new vendor. We electro-etch our part number on this part. We have never had any problem with this operation. On this batch, instead of taking about 1 second for the etching, we take over ten seconds and don't get a good part number. The only difference I can see is the heat treat and black oxide vendor. The material is 4130 steel tubing, heat treated to 83-86 on the N15 scale (50-55 Rc) and given a black oxide finish. We usually induction heat treat this part, but this time it was hardened in an oven in a 35% Carbon Dioxide atmosphere. Is it possible that the surface resistance of the steel has changed somehow?

Thanks-

Roy - Another possible explanation is that the black oxide shop is using a black oxide that has better penetrating abilities and/or is leaving them in the bath longer to form a thicker black oxide layer. Check with the vendor to see if they process differently than what you were accustomed to.