Byzantine succession problems

[unmerged(616886)]

I was playing through a game as a Byzantine Emperor (myself), and after giving my eldest son a duchy he stopped being my heir, and I do not understand why. I have Agnatic-Cognatic Primogeniture, which you start with. I noticed that the Doukas, who is the basic Emperor, has the same thing going on. Can anyone tell me what is going on and how to stop it?

Sorry if this is an old forum question, and thanks for any help.

 

[Sarog]

Your eldest son was probably born before you became emperor, and was bumped from being heir when a new son was born. The Byzantines use the Born in the Purple trait for imperial succession, so that kids who are born after you become emperor come before kids who are born before.

 

[Mackus]

You can grant any of your sons who are without born-in-the-purple trait, honorary title of despot, and he'll be your primary heir again.

 

[fcmsaab]

Is your son a bastard?

 

[Lorehead]

You can grant any of your sons who are without born-in-the-purple trait, honorary title of despot, and he'll be your primary heir again.

Thank you; I did not know that.

 

[unmerged(616886)]

Thanks guys, that makes a lot more sense.