Make sure ALL the legacy option of off!
We had this issue with our new HP EliteDesk 800 G3's and in the end I found that we had to make sure the legacy option were all disabled.
We have secure boot activated and if this is on and you can still see a tick in the legacy options but greyed out then you need to remove secure bott, remove the legacy options and then re-apply secure boot!
Took ages to find that one.