Canon FD to EOS converter? Confused?

I bought a FD to EOS converter online, knowing very well these types of things are hit or miss. I bought it primarily to use with a older, but at one time top of the line, 75-200 telephoto lens. I have used it quite a bit, and the telephoto captures every bit as good of images as a dedicated lens (with the exception of the aperture limitations in some situations). Images have so far been sharp as a tack with no visual distortion using this particular lens and the converter. This morning, I decided to play with some other FD mount lenses I have. They don't seem as willing to produce good images. It reminds me very much of the "grainy" appearance of high iso film, and the wide angle got a white, hazy almost "vignetting" thing going on. Any ideas as to why one lens works quite well with the converter and others don't? Possible the higher quality optics in the telephoto vs the other lenses? If the converter didn't work at all, I would understand. If the converter worked well with all the lenses, I would understand that. But I'm not understanding whynit only works well with the one lens. Anynideas/insight?