2 versions of the Asus p8z68, pro and non pro @ newegg. First, have you looked at the picture of the Motherboard (ie is it possible that they forgot to install) and have you read the installation procedure in the manual.
That said, I've had some that took more pressure, Than I was comfortable with, to get a good seat - even with locking pins at both ends. On new builds, I install memory while the MB is not in the case - have it on the kitchen table so that I Don't have to "sweat" the flex on the MB - mind you, that method does not help if replacing ram and MB is already in case. This is right after I install HSF, which I also do on the table.
Normally I do the complete build on the table, Just the bare bones, enough to be able to power on and go into bios to verify system health. Then I add in a dvd drive so that I can run Memtest 86 for a couple of hours.
Once I'm sure everything is working, I then install into case.
it worked!!!! you are the master (chief)