Normally when I flash an SD card and insert it in my raspberry pi, it comes online straight away.
This stopped working. They do not come online anymore. I only have 4 other devices. Any ideas on where to start debugging this? (yes, the wifi password is correct :))
To be clear, your 4 other devices are online? Are you flashing the same image in the SD card as the previous ones? What Raspberry Pi version are you using? (PiZero/1/2/3/3B+?). What OS version is it?
Could debug things as:
check that the image is appropriate for the given Pi version
connect the device with an Ethernet cable, which should not rely on wifi, if that is an issue
can flash a development version image, and then you can either use a USB-to-UART cable, or this PiUART to connect to the device over serial, and see any logs there
These are some ideas, let us know if you have a chance to try things!