If you click on the province, then on the name of the province in the province bar, you can see a list of what's affecting morale in that province. It'll show you what the target morale is, and what's positively and negatively affecting target morale.
If the target morale is 100+ then the province morale with rise a little each day change to meet that. If it isn't 100+ then you need to build more factories/harbors/railways/fortresses (only these 4 buildings help morale) and make sure you always keep at least one full day of every resource in stock for every day change.
If none of this helps and you think you may be experiencing some kind of bug, feel free to report it to support, found in settings.