This is not a new problem
I and others have had problems with Crucial RAM on the new late 2013 iMacs. I was warned on another thread and knew better than to push hard. So when it didn't fit I returned and bought RAM from OWC, which fit perfectly.
Both I and the OP of that thread were ridiculed by a number of posters on this forum even after posting a picture of the RAM. Find the thread under my profile if you'd like, but it won't help you now.
My suggestion is to let an Apple Genius take it out and then return it and buy the RAM from OWC. They are selling Hynix RAM, which is the RAM Apple is installing in the new iMacs now. Good luck.