In Witcher 3 experience points given as reward for completing quests can be modified. You can't assume that for completing the very same quest at the beginning of the game and later in the campaign you will receive the same amount of experience points.
Two main factors that determine how much experience you will receive are current character level and suggested level for the quest. You can check the second parameter anytime you want in your journal. Suggested experience levels can be found next to the information about the location in which the quest takes place. Red color means that the task is meant for character with higher level. Green means that Geralt is close to the recommended level. White color means that the task is for character with lower level that the one you have. Because completing the latter ones shouldn't be any trouble for you, the game might limit or completely remove experience given for completing them.
How to avoid this problem? You can't completely avoid the "loss" of potential experience points, however you can greatly reduce the amount of such situations. Try to focus mostly on quests that are more or less suited for your current experience level. Leave the more advanced quests for later, even if completing them wouldn't be too hard (because of, for example, the lack of battles in them). Additionally, try to explore the world in similar order to the one planned by the developers. Start with Velen, but avoid territories where you can meet much stronger monsters. Then, at around 10 level, go to Novigrad. At around level 15, travel to Skellige islands.
Important - some quests don't have information about suggested character level. Don't worry about it, those are unique quests about, for example, playing Gwent card game.
Important - the scaling of experience described above affects some other activities for which you gain experience as well. It includes securing some optional locations (the question marks on the map).