I guess you overexposed the UV-sensitive film. Overexposure occurs when stray light (caused by the carrier film) starts exposing the traces.
My suggestion: Use a small piece of PCB-material and expose it no longer than 240 seconds to test for best results. (240 seconds is a good time when using six UV-fluorescent lamps)
If the (fine) traces don't disappear check if there are any leftovers (clouds) of the film. If so, increase exposure time slightly to 270 seconds.
To check for clouds dunk the PCB into etching solution for 30 seconds and rinse. The copper to be etched should look dull all over the board.
Boncuk