The problem is likely to be in the exposure and developing stage:
1.The black areas of your artwork are not opaque enough. Hold it up to a window. Do the blacks go grey?
2.The image has not been printed in reverse. Always print it back to front so when it is turned the right way round the ink/toner is touching the photo-resist.
3. There is poor contact in the UV
4. The UV exposure time is too long (3 mins max.)
5.The developer is mixed too strong. Check the label
6. The developer is too hot. Check the label – if in doubt use at room temperature.
7. The board has been left in the developer for too long. Developer for 15 seconds; rinse and inspect and then develop for another 10 seconds if need and rinse and repeat if necessary.
8. The board was not rinsed to stop further developing as soon as it was removed from the solution.
We have a video on our web site that takes you through allthe above