Rendering transparent plastic materials

Hi,

I'm having a problem with my transparent plastic materials. The render comes out darker than what shows on the model and the "appearance icon" for the material.

All other materials in the model looks fine in the rendered image, but the plastic transparent materials seem to need so much more light.

I have the following settings:

Render Setup Quality: Maximum

Anti-Aliasing Quality: Maximum

Shadows Accuracy: Maximum

Enable Final Gathering: On

Enable Global Illumination: On

Enable Caustics: On

Set shade quality to: 30

Edge display quality: Very High

Anti-aliasing : 16X

I'm working with Creo 3.0 and ARX.

I hope you can give me some advice, cause I'm going crazy trying to make this work.

/Eve