Micro pave going black when rhodium plated

Looking for some advise please.

An all white gold ring.

Micro pave set with approx 40.015ct dia’s. After resizing we emery
the entire ring to get all the old plating off. It is hollowed out
in the back this is not touched. When we re-rhodium plate it the
stones come out black. It doesnt look like the actual diamonds go
black but the metal behind them. To get rid of it we gently heat the
ring with a torch and its gone away after it comes out of the pickle.
There are 2 rings of a similiar description that have the same
problem. Any ideas on what might be causing this? We keep our
solutions very clean and it even happened with new solution.