Q: Macbook Pro not able to use the iMac display

Hi - I am trying to use my iMac monitor as the display for my Macbook pro and can not get it to work. I have a Macbook pro with OS X El Capitan 10.11.3 and iMac with the same OS - I connected the two via thunderbolt cable and used Command F2 as instructed in another post (albeit referencing an earlier version) and I can not get the Macbook pro to use the iMac display (both are retina displays).  Any suggestions???