The delay is most likely caused by the "security background check." You have two options:
1- Keep waiting
2- Set up an infopass appointment to find out what is causing the delay. An infopass appointment is an appointment you set over the internet and you go to the nearest USCIS office to you to ask about the status of your case. to set up an infoass appointment you can go to the following website:
3- you can hire an attorney to sue the USCIS for you and you will receive your naturalization appointment within few weeks. This option cost you some money.
All the best.