Stoneware clays are not completely vitreous. If the clay is not fire up to it's proper temp, that can increase it's porosity. So a cone 6 body that only reaches cone 4 is more porous. Also, if there is a glaze flaw, you can get leaks. The thin spots you described could be the problem but a glaze that doesn't 'fit' the body, meaning there is some crazing (cracking in the glaze), may be the problem. You might try re-glazing the interior and firing the piece again which may or may not help. Also be aware that putting a really really thick amount of glaze in the interior can exacerbate the crazing issues. In this case, more is not better, The correct application is better. So fire it again an see what happens. We learn from our errors.