If your shield rank is more than 5 ranks below or 10 ranks above the recommended level of the leve (ie: your shield is rank 9 and you are doing rank 20 leves) the SP reward for it gets reduced just like your normal class does if it's out of level range.

If you want to get your shield caught up, try leveling a mage class with a shield (assuming you haven't worked on a mage class yet). Barring that you can most likely just grit your teeth and bear it. Leveling up eventually slows down and that might give your shield rank a chance to catch up anyway.