a landscaper planted some Pacysandra under by River Birch trees that face West. the plants that are facing North are doing great, however, the ones facing west which are under partial shade have yellow leaves and not flourishing. I'm thinking the problem could be either (a) they didn't amend the soil properly and/or (b) too much sun. He planted these about 20 months ago and the yellowing didn't happen until this Spring. I'd hate to see them die or waste them, any suggestions

Those ground covers do like shade, but you say they are in partial shade. If they get a lot of sun in the afternoon, that could be the problem. I wouldn't think it would be the soil. Check on them in the p.m., after 4 o'clock or so and see where the sun is hitting them.