Hi, its been a while since I have looked at this. I did manage to solve the issue and get it to add a new row and save.
Where you click the new drop down do you see your UD table there? Also check if any fields in the UD Table are required.
I managed to solve this on topic Add UD child table (nothing populating) and also share my code. Let me know if this helps you?
I do remember when going through the wizards where I link them I clicked next so I could enable the create new then click back to then enter the link. Not sure if this makes a difference.
Regards
Kirsty