If you find yourself not gaining XP from monsters that you kill, there are a few things you will want to check.
First, check to see if you are seeing this symbol when in a particular map:
If you see that symbol, it means that you are too high a level to receive XP from this map. Tapping the badge will bring up this window with more information:
If you are not seeing that symbol, then you will want to check and make sure that you do not have the Disable XP option turned on. To do so, open the game menu and tap on Options
Then tap the Social Options tab and make sure the Disable XP Gain checkbox is unchecked.
If you have checked both of these things and you are still not receiving XP, then feel free to contact our Support Department by click on the Contact Us image located on this page.