Several factors can be at work here, the biggest thing is how much of a drop in HP/MP a character gets, the vitality stat also seems to affect some things as well as I've noticed the lower vitality a character has, the smaller their HP growth is.

To raise Maria's MP, I would cast her lv. 8 spell as a single target spell so you can blow through her mp cause if she's finishing fights in one round with a spell, its not going to go up very often. As for the HP, you need to knock her into a near death state (where she's crouching) to consistently see HP growth if her Vitality stat is low. The easiest way to level HP/MP is with the spell Swap, which is obtained in Mysidia which is south of Altair and will require the canoe to reach. You are not suppose to get here until much later so the monsters will probably one shot you so prepare to run and use spells like Warp and escape. If you haven't freed Fynn yet, you might want to save and try your hand on the the royal guards. Higher level enemies will net you more bonuses than trying to fight weak enemies. Having level 8 magic and high bare handed weapon can allow you to kill one but expect to watch him kill a character every round. They drop some good spells and weapons so farm them until your comfortable and then make the journey to Mysidia to get the Swap spell. After that, its just about leveling the spell until it has a decent success rate and then spamming it on weak imps.