VSS/TSS ratio not affected by SRT

Hello, I'm a novel user of DHI West. We are using the software for the design of an urban WWTP. The client is interested in the relation between de SRT and the stability of the sludge in the tanks expressed as the VSS/TSS ratio. The problem we are experimenting with the model is that VSS/TSS ratio is barely affected by de SRT, or it is in an unexpected way (it should be lower al higher SRT valuers but in some simulations it goes the other way around). The model is build using the ASM2dMod library. Thanks in advance, Lucas Benac