I have a problem similar to this one. I have played KOTOR through before and done the Carth romance, I used the Carth romance mod and had no problems with it.
I've been playing the game through again recently and have been following the same guide as the OP. I have reached conversation 10 BEFORE getting the fourth star map. I then picked up the fourth star map. However I am now trying to talk to Carth after getting it (and before getting captured by the Leviathan) and he isn't saying anything. It's been a long while since I played the game the first time so I have mostly forgotten if this is normal.
Am I supposed to do the Leviathan level before I can get conversation 11 + 12? Because at the moment Carth isn't saying anything more to me after getting the fourth map, and I definetly reached conversation 10 before doing so.
Any pointers would be appreciated, I hope there isn't something wrong with my game.
EDIT - I have done Dustil's quest.