Maximum mana is based on your essence; if you expended essence (revitalizing land so a city can be founded, losing and escaping from a battle, saving the wolf cubs, etc) you'll have less max mana. Solution: get more essence when you level up. This is entirely separate from spending mana on spells, which will then regenerate on its own.
Not sure that's clear, let me give an example just to be sure:
You have 20 essence, therefore you have 20/20 mana. If you spend 5 mana on a teleport spell, this reduces your mana to 15/20, which will eventually regenerate back to 20/20. If you spend 5 essence on a revitalize land spell so you can found a city, this reduces your essence to 15 and therefore reduces mana to 15/15, which will not be recovered. But next time you level up, you could put a point into essence raising it back up to 16, which would bring mana to 16/16.
Practically, this means that revitalizing land seriously limits your ability to cast more powerful (higher cost) spells. With a little searching you can actually find spots of already revitalized land to found a city for 'free', and once founded your cities will spread revitalized land where more cities can be founded. So be careful where you actually spend the 5 essence to use the revitalize land spell, save it for crucial spots where you just have to have a city.
