Coupling of Transport of Diluted Species with Darcy Flow with thing layers

Hello All,

I'm working on a model of blood flow through a porous media. The blood flows through two main conduits an arteriole and a venule both of which have a main luminal region as well as a thin layer in which the reactions and such are slightly altered. Flow through the capillaries is modeled as Darcy flow through the tissue. So first I solve the Darcy flow with no issue, and then I attempt to solve the transport of diluted species physics and run into issues with regards to solving. Ideally I'd like to take this dynamic with a change in conduit geometry in response to mass transport, but can't do so due to difficulties in solving the steady state problem. I'm wondering if the issue lies with the coupling of Darcy Flow to the mass transport or if its due to the thin regions located within my two conduits? Any advice? I'm attaching the model as well as a picture of my solution convergence.

Thanks

Pat