I know!
Sometimes you cant even photograph it. Its just the way the light hits it that shows it, and sometimes you only see bits and pieces, like the dot over the " i " which makes you dip and turn the piece every which way, until you can see the actual signature. Thats how I found out I had a Venini signed cornucopia designed by Bianconi!!!
This has actually happened to me 4 times already. You just have to really look, even if at first you dont see a thing. Different reflections of light will bring out the signatures too. I used to just check under daylight, but I found out that is not really the only way. I also tried putting the pieces under a normal house lamp and that has helped me take better photos of the signatures. Yes they might look washed over and yellow, but you will probably have a better detailed signature than in daylight. Sometimes the glare of daylight can be too overpowering.
Also, if you have a digital camera with zoom, this will help alot! You can really study the bottom of a piece under the glare and not go blind by trying too hard, lol.
Javier