Convergence problem with time-dependent inlet velocity

Hi, I'm simulating a coupled Laminar Flow and Transport of Diluted Species. When I put a steady inlet velocity the simulation workes perfectly. But when I change the inlet velocity to a time-dependent one (let's say U * t[1/s]) the Laminar Flow converged if solved individually but the coupled Laminar Flow and Transport of Diluted Species does not convenge. Even if I solve the Laminar Flow first, and then use that velocity field in the Transport of Diluted Species as the convection flow, and try to solve that individually, it would not convenge.

Thanks in advance for your advice on the issue.