Looks like mine may not be empty . . . I can't explain it, but the Hyatt I was at said they were bad. CardCash says not. I called Hyatt's third party processor, and they appear to be good.
Strangely, cards I bought on Amazon worked just fine at the Hyatt. But not these from CardCash.
So, CardCash may be off the hook here, but it's a mess because now I have lots of GCs that will be out of their 45 day protection before I can use them.