You’ll need to play in a campaign appropriate for your level. Forest Haven is for levels 1-10, Dark Forest is for levels 10-15, Balefort Castle is for levels 15-20, and so on.
For example, if you are over level 10 and playing in Dark Forest, you probably won't get experience.
Also, please ensure that "Disable XP Gain" is not checked in the options menu!
To check this option, follow these steps: