Some explaination of what sort of shadow probles you are having might help us answer your question. Sometimes just increasing the value on the 'Shadow' dial of the lights will help, for an infinite light you will need to double click on the dial and reset the limits to a value grater than one. The more geometry there is in the scene that has casts shadows enabled the greater will be the attenuation of the shadows, for this reason it it offten halps to turn 'Casts Shadows' off for the ground plain or background prop you are using, but there are also circumstances where turning it on can help with shadows, experiment. Zooming in with the shadow cams offten helps as well. Objects with some thickness offten receive shadows better than plains. Putting a Poser square under the floor prop sometimes helps with missing ankle or foot shadows (don't know why). And remember that in Poser 4 a light that is parallel to a plain will not cast shadows on that plain, tilt it slightly towards the plain.