Chris:
Well, you're going about solving the problem correctly. Most people aren't aware that the urine of all mammals fluouresces under ultra violet light, but the fluorescence is not intense, like a 1960's Jimi Hendrix poster, but really quite faint. And, I've heard a lot of good things about Nature's Miracle, so you're using the right cleaner, too.
However, the problem MIGHT be that the black light source you're using isn't powerful enough to make the dried urine fluorecsce sufficiently to make it noticable. I would phone around to the places listed under "Janitorial Equipment & Supplies" in your yellow pages phone directory to see if any of those places in your area rents black lights. You should also try the Party Supply places because black lights are a popular item for Haloween parties. The tool rental places are worth a try, too. And, if push comes to shove, most professional carpet cleaning contractors will have a black light, and I'm sure that for a fee they will come out and point out the urine stains to you.
Just in the same way as this Q&A forum caters to DIY'ers, there is a web site that caters to the Janitorial Services sector of the economy, and it has a Q&A forum where you'll be able to talk with professional cleaning contractors who would be more knowledgable about removing pet urine stains and odors:
'Cleanfax - The #1 information source for carpet cleaning & restoration professionals.'
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That web site is owned by CM Magazine, which is a trade publication for the commercial building cleaning industry, but the content of the web site is heavily weighted toward carpet and upholstery cleaning. Many carpet cleaning contractors hang around on that web site, and all of them would have had experience dealing with pet urine stains and odors. You may have to register on that web site to post on their Q&A forum.