- ensure the lid is actually closed (this happens regularly) 

- ensure the FFC is connected correctly 

- ensure the cables are connected as shown in the guide:


You can also start the PSX with the lid opened, go into the BIOS menu and then close the lid. 
The controller inputs should become non-responsive then, as the console recognizes that there is a new disk.
If all signals are connected, the loader will then start up after a short while.