It sounds to me that you are using a UV light source that also transmits visible light; this will not work. Here is some information.
"Uses of Blacklights
(Long Wavelength Ultraviolet Lights)
Fabric and Textiles
Textiles: Thread and Cloth: Synthetic fibers--rayon, polyester, etc. made since WW2 all fluoresce. This allows you to detect new military uniforms, new quilts, new doll clothes, etc. Even if old fabric is used to repair an old quilt, the new thread will fluoresce. A quilt from 1800 is less likely to fluoresce, while a quilt made in the last decade probably will. Mass produced laundry detergents from the current and past decades have contained additives that make your laundry whiter and brighter. These additives can make textiles fluoresce under a black light, which would lead a person to believe an antique quilt/textile, washed in these detergents, was newly made."
Search eBay for "Long Wave Ultraviolet". You do not want an inexpensive lamp because a cheap lamp will not cause the synthetic fibers to fluoresce strongly enough to be readily detected. Spend $50 or more is my suggestion.  When using your long wave UV light do so in the dark of night or a room with no windows. Turn on your UV light and turn off ALL other light sources. Then look at your patches - front, back AND edges because sometimes the base fabric will fluoresce even when the embroidery does not.
