Let me try another approach.
If a dowser claims a 100% success rate, is tested and can't meet the protocol of the challenge, they have failed.
Should they be retested? Resources permitting, of course they should.
Why? Because belief in dowsing still persists. All we can do as skeptics is say "Show us, and we'll believe." If we stop saying "Show us," we lose.
Many dowsers have failed, even after an open test, and have claimed it was sunspots, electromagnetic interference or whatever. I seriously doubt these claims, but since I am not that person, I have to give them the benefit of the doubt. And that doubt is the doubt of possibility, not likelihood. I've seen nothing to think that dowsing was real.
We can conjecture and posit that the person might be lying, or be seriously deluded or whatever. But the only tool at our disposal is another test.
Now, if I notice during the dowsing test that the rods tend to bend towards lights, I might note that as "interesting." If the claimant thinks this is significant, they could decided to have another test. In fact, they can decide to have another test for any reason at all. It's up to the organization to decide whether they're willing to test or not, and that's not a subject I've addressed.
To be clear: I'm not faulting the IIG or their conclusions. Their stated results are exactly correct. There is NO ambiguity in the results at all, as is proper. However, what I observed was interesting compared against other challenges. And if VFF feels she'd like to be tested again, I think that's fine.
Sigh, one last thing... though I'm probably opening a whole new can of worms.
One thing that shouldn't be considered during a challenge is mechanism. Precautions should be taken to make sure known mechanisms are not used, but as for what paranormal method the claimant uses, it's irrelevant to the test. Conjecture about it should be avoided when designing a test. The goal is for the claimant to be satisfied that they can perform under the conditions stated and that trickery and chance are accounted for. That's it.