Okay. So I'm playing as Ireland, everything works fine, I take over Dublin (early campaign) and my wooden castle can not retrain my Ostmen and Hobiguir units. It says there are no available units. I have 29 Ostmen left (out of 60), and 19 Hobiguir's out of 30.

If I disband them, I can train new ones. Regardless of how many turns I wait, the option to retrain them does not appear, and I should be able to retrain units that belong to that castle.