Thank you @Loni for clarifying the matter.
Unfortunately I couldn't find information about how long you have to wait until you can travel to Kosovo again for 15 days after having been there once before. You should contact the Kosovar embassy in order to find out more about this topic.
But have you thought about getting a visa for Kosovo? It would enable you to stay longer than 15 days. You can use the following website to find out the exact requirements (depending on your country of origin) and where you need to go to: