I don't know if it only happens to me but I can only find pigman. Could it be a bug or certain seeds don't spawn ghasts?
I've tried looking for them around the nether and have not found any.
FYI – My server.properties option for monster spawn is set to true. No peaceful mode is off and it's a SMP map.
How could that happen?
SCREENSHOT AS REQUESTED
So as you can see only pigman in sight.
Best Answer
Get out in the open
Start digging tunnels and look for a BIG AREA (the ones that involve lots of lava).
A Ghast can spawn in a small area but the chance is very small compared to a big area...