I am not completely sure what you are trying to simulate but if I am not wrong it is a block at an angle of 45° degrees (angle of attack) right?
If it is so, most likely the reason your solution does not “converge” more is that it cannot converge more. You are performing a steady-state analysis of a box at 45 degrees of AOA. There is flow separation. The solution does not have a steady-state solution but a number of “solutions” that come close by, because the phenomenon is inherently transient. Therefore, your results are called “quasi-steady state”. The oscillations you see in the residual plot is the solver trying to come by with a solution that does not exist. Image a simulation of an airfoil at 18° AOA. There is flow separation. The airfoil is at stall. There’s buffeting. This is a transient phenomenon. Using a steady-state solver on this problem will cause the solver not to find a satisfying converged solution.
Solution: Simulate using a transient state approach.
Do not try to (not recommended): Use the quasi-steady state solution (the one you already have) as the definitive result. If you are measuring lift and drag, you will obtain different results with the transient approach.
@SBlock, my suggestion is for SimScale to write a tutorial / technical text about this behaviour. This should be like the 10th time I see this issue addressed in the forums 
Best regards,
J.A. Gutiérrez.