I have planted pentas in my yard several times, and they always die. My friends have no problem with theirs. The only problem I can think of is I noticed we have a lot of clay in our soil. Could that be the problem? Thanks.
Try mixing several inches of compost into the soil. Make sure the location is bright but not in too much direct sunlight. Keep the soil moist but not too soggy wet. Try to not wet the foliage too much when you water to minimize leaf and stem diseases.