So, ranking on Resident Evil 5 is based off 4 different things. Your Accuracy Enemies routed (killed) Deaths Clear time

You do not have so get S on all 4 in order to get an S, an overall S is still possible as long as you get: 3 x S and 1 x A 2 x S and 2 x A 3 x S and 1 x B

So, here are the things needed to get an S on each category per sub chapter.

Chapter 1-1 Accuracy: 70%+ Enemies routed: 20+ Deaths: 0 Clear time (Less than or equal to): 12:00

Chapter 1-2 Accuracy: 70%+ Enemies routed: 15+ Deaths: 0 Clear time (Less than or equal to): 13:00

Chapter 2-1 Accuracy: 70%+ Enemies routed: 60+ Deaths: 0 Clear time (Less than or equal to): 23:00

Chapter 2-2 Accuracy: 70%+ Enemies routed: 50+ Deaths: 0 Clear time (Less than or equal to): 22:00

Chapter 2-3 Accuracy: 70%+ Enemies routed: 40+ Deaths: 0 Clear time (Less than or equal to): 9:00

Chapter 3-1 Accuracy: 70%+ Enemies routed: 30+ Deaths: 0 Clear time (Less than or equal to): 18:00

Chapter 3-2 Accuracy: 70%+ Enemies routed: 50+ Deaths: 0 Clear time (Less than or equal to): 19:00

Chapter 3-3 Accuracy: 70%+ Enemies routed: 30+ Deaths: 0 Clear time (Less than or equal to): 19:00

Chapter 4-1 Accuracy: 70%+ Enemies routed: 50+ Deaths: 0 Clear time (Less than or equal to): 23:00

Chapter 4-2 Accuracy: 70%+ Enemies routed: 35+ Deaths: 0 Clear time (Less than or equal to): 16:00

Chapter 5-1 Accuracy: 70%+ Enemies routed: 7+ Deaths: 0 Clear time (Less than or equal to): 12:00

Chapter 5-2 Accuracy: 70%+ Enemies routed: 40+ Deaths: 0 Clear time (Less than or equal to): 22:00

Chapter 5-3 Accuracy: 70%+ Enemies routed: 30+ Deaths: 0 Clear time (Less than or equal to): 38:00

Chapter 6-1 Accuracy: 70%+ Enemies routed: 40+ Deaths: 0 Clear time (Less than or equal to): 26:00

Chapter 6-2 Accuracy: 70%+ Enemies routed: 35+ Deaths: 0 Clear time (Less than or equal to): 23:00

Chapter 6-3 Accuracy: 70%+ Enemies routed: 25+ Deaths: 0 Clear time (Less than or equal to): 22:00