If you are not able to include any app in a folder on your device, you might be trying it the wrong way. Sometimes, it becomes a little annoying trying to include a particular app in a folder. You need to drag and drop the app in the middle of the folder so that you can make it a part of your folder. Besides this, every folder can contain only a definite number of apps so if you are not able to include an application in a folder, you must check the maximum number of apps that it can accommodate.