Corn is notorious for creating stuck mashes as it is incredibly gummy.
If you are using high proportions of it, we would definitely recommend using rice hulls in the mash and possibly incorporating a beta-glucan rest (eg a 20-minute mash step at 45°C (113°F) before raising to your saccharification temperature).
This should help to make the mash less 'sticky' and allow for better recirculation.