Kent states that higher potencies will correct problems raised by lower potencies. Following his advice I have often used Nat mur 10m/cm to bring a case back to square one after it has been "spoiled" .
It is reasonable if you accept that the higher potencies have a stronger relation to the mental sphere. If your case has physical symptoms and you have used 30c then it may be that the physical are related to mental problems - in which case it is better to use a higher potency.