Nokia Lumia 521 - Why does the side key seem to get stuck after I put the back cover on the Lumia 520, 521, 525 or 526 phone?

The side key (camera key, lock key or volume key) could get stuck under the back cover. Remove the cover and push the side key from inside of the cover to outside to make sure it is in the right position. Put the cover back and try again. Be careful that you do not touch the side keys when placing the cover on the phone.