Sometimes a run doesnt populate cleanly in the center of a stroke, but you can use the Center in Stroke function to help.

Press "Shift+C" to center the selected modules or the current run within the stroke of the letter.

Here in this example, there is a kink in the center path for this stroke. Just select the modules and use this command.

Also remember that holding the ALT key while in PowerFlow will place the cursor in the center of the stroke if you want to manually drag the path or place modules individually.