I don't know how you're generating your functional equivalent of a negative. If you're using software to generate your "negative" or equivalent, try generating a "pour" GND or Vcc-connected plane on the part of the board that you don't otherwise use. The less material you have to etch off the plated board, the easier it will be to control the etching.

If you're drawing your traces on the board with a pen of some sort, be sure you're using a truly opaque ink. When I did this sort of thing with a plotter, I had little success until I started using a latex-based black ink. In reality, yellow works about as well, since it's UV that you're trying to control, but density is a critical factor, and yellow is hard to see. It all depends on the type of photoresist you're using, of course.

RE