The problem is likely to be in either the exposure or developing stages and could be:-
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.