Did you changed the Terminal to send the Output to the SERIAL-Port?
You need to do this in the Bootloader of OpenBSD (as example, for other OSs I don't know it).
stty com0 115200 set tty com0
And then enter "boot" to boot further from your Media.
By default any of your OSs will send the Output to the non-existing Monitor and not to the Serial-Console.
So for your it looks like the Boot-Process stops since nothing is send to the Serial-Console anymore. :)