If you use oracle designer to generate your form then you must set the preference as follow:
The preference ‘End User Interface’ -> ‘Block Sync – Co-ordination scope’ must be set on ‘Any visible block in current form’.
If you use oracle form builder then set the coordination style on ‘F’.
CGBS2$.SET_COORD_STYLE( ‘F’ )
defined in ‘when-new-form-instance’ trigger.
This should be done automatically when you have created a relation between the master and detail blocks ?
Have you done that ?
What Forms version are you using ?