Is it possible to have a WF take into account the change of stage in the Business Process Flow?

Hi,

I am trying to get a WF to modify a field value in the opportunity record according to the Business Process Flow stage.

The problems seem to be that the WF doesn't detect the change in the BPF stage. I used the field "Sales Stage" and "Process Stage" as triggers but it doesn't work. 

Does anyone as any idea what might be the problem?

Thanks,