Normally you need to calibrate the exposure time/develop time by exposing a strip of pcb in steps. You start to expose a part so many minutes, and then move the film a bit and expose all, doing this with timed incrememnts you then develop the strip until one of the sections are perfect. Then you know how long to expose and how long to develop. Or you can set the development time fixed, and find out the exposure time from the best part of the strip.
To get a perfect result every time you also need to keep the development solution mix exactly the same every time.
There could also be some tradeoff between lamp intensity, film thickness and opacity, to get the finest details of your PCB perfect. The film side should be on the side closest to the photoresist.