Hi. My name is*****'m a camera repair technician with over 20years in the business and I'll be glad to help you with your question today.
This is usually caused by a small piece of plastic that has broken off the front of one of your SD cards. This becomes lodged under the contacts and will prevent another card from inserting completely and locking in place.
This really isn’t a user serviceable problem and should be repaired by a qualified technician. You may want to check locally for a camera repair shop in your area, or, if you need assistance finding a shop, please let me know and I'll locate one for you.