Binding sources and combo boxes problem

There are five tabs in a form. (Windows application). Each tab is the subcategory of the tab before it. (Tab 2 is the sub of Tab 1, Tab 3 is the sub of Tab 2 and so on). There are two combo boxes in each tab, starting from Tab 2. Tab one has one combo box only, and has several 'values' binded to it using Binding Source 1. The first combo box in Tab 2 is the parent for the second combo box in same tab.(It displays 'values' from Tab 1 using Binding Source 1).The second combo box displays the sub 'values' of the first combo box when selected. This is done via Binding Source 2.

Although I have done all these things for Tab 3 onwards also, with the appropriate names for the Binding Sources, Tab 3 does not display the values in the second combo box. Tab 4 and Tab 5 don't even display values in the first combo box AND the second combo box.

Why is this and how do I correct it?