You have specified a total flow rate for all cooling circuits. The solver calculates the pressure and temperature distributions based on this input. This boundary condition needs to couple the flow solutions of all the circuits in order to calculate the pressure drops in each circuit for the specified flow. This is the most difficult network solver boundary condition to satisfy and may not be achieved in certain cases.
Check the Reynolds numbers of all the circuits. If any Reynolds numbers are greater than 100,000 then the specified total flow rate value is likely to be too high. Try reducing the total flow rate value. The variation in the dimensions of the various cooling circuits may be too large. Ensure that the diameters of all the circuits are within a similar range. If the residual is small compared to the specified flow then the warning could be ignored. The warning could be due to modeling errors as well. In this case test the circuits with the default settings to ensure that each circuit solves on its own before specifying a total flow rate for the cooling system.