I don't know what "pf paper" is. You should consult the rep from the ink manufacturer as to what paper you can print on. Your ink must be matched to the substrate or you will get all kind of undesired results. The ink rep will want to know the basics of your work, like what line screen, what mesh, the thickness of the substrate, coated or uncoated stock and maybe more before he could answer that.
You are either
1.Under-exposing : in which case most of the emulsion will wash off along with your design.
2. Over-exposing: in which case the design would not wash out of the screen it would be set along wiht the negative area.
Things to be sure of:
1. Is your photosensitive emulsion out of date ?
2. Are you being careful not to expose the coated screen to any other light source ?
3. Is the emulsion applied evenly and thinly using the correct application trough ?
If the last three categories ar ok, then it would appear that your problem lies with the exposure time. Do small practise pieces (they don't have to be stretched for this purpose) and experiment, writing down the exposure times and results as you go.
Good luck with that.
If after all that you are still not getting a good result, why not pay a small fee for another company to make up the screens for you ?