While it is fully built remove the motherboard battery for a few seconds and then try, somebody else mentioned this and over the past 24 years I've had to do this at least 5 times for a system to recognise the graphics.
It doesn't work for an assembly line but I always build computers out of the box. The motherboard sits on the table and I fit the CPU, fan, memory, PSU and monitor straight out of the motherboards graphic output and see if it all turns on and goes to BIOS. I then add the other parts 1 by 1 and each time see if it boots to BIOS. I then add the OS and drivers and only put it in the case when its sat on my table working for 48 hours.
Example: