Having trouble getting Target Display mode to work with Apple's Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter with new MacBook pro?
I have had the same problem with my 13" 2018 MBP to my iMac 27-inch, Late 2012.  Talked to Tech support for weeks at the advanced levels.  They told me they are working on it and look for a firmware update for both the laptop and the iMac.  Bought my MBP when they first came out, it is now September 3, 2018 still no update. Still does not work.  Have not heard of anyone finding a work-around.  I used to use target display mode with my MB-air and was looking forward to using it with the new high-powered laptop (it was NO-GO).