Mark increases are dependent on the monster's danger level like Pumpernickel said. You also get only a set number of increases from a single monster. For example, if you block three attacks from a rat, you gain 3 shield marks (1 from each block), but if you block three attacks from a large orc, you gain 9 shield marks (3 from each block). Similar things happen for weapon skills.
I said it before, and I'll say it again. If I knew scripture like you, I'd prolly be an athiest too.. -gut
/l、
(゚、 。 7
l、 ~ヽ
じしf_, )ノ