In the northern hemisphere, the maximum charging power is obtained when the panel is facing between South East, and South West, ideally South.
If your windscreen is facing North, mount the panel on the side windows, it will make a BIG difference to the effectiveness.
If the solar panel isn't making a noticeable shadow, then it isn't generating a noticeable power !
If you want to see how solar panel generation varies throughout the day, checkout
http://www.gridwatch.templar.co.uk/, the red solar generation output for the UK (as well as nuclear, (combined cycle) gas turbine, coal, wind, and power imported from France, Holland, Ireland et al.