In this particular case please make sure you are using the correct type of RAM memory on your NUC5i5RYH's which are not displaying any image at all. You may get more information about tested Memory here Intel® NUC Boards and Kits — System Memory
For the ones that are not properly displaying the BIOS screen or showing Blank screen when accessing BIOS I suggest checking the Cable you are currently using, if possible test another High quality cable. The supported video outputs are Mini-DP 1.2 and Mini-HDMI 1.4a . Intel® recommends high quality cables and straight connections in order to ensure best graphics performance on your NUC.
You may find relevant information here Intel® NUC Boards and Kits — Tested Peripherals