I have the extactly the same problem.. I am creating a control where a
collection property is exposed which has a nested collection object.

It looks something like this once i enter data through the default
editor (in the design view)..

<COL_A>
  <A1:COL_A_ITEM Name="Test" Index="1">
      <COL_B>
    <A1:COL_B_ITEM Visible="True"></A1:COL_B_ITEM>
    <A1:COL_B_ITEM Visible="True"></A1:COL_B_ITEM>
      </COL_B>
   </A1:COL_A_ITEM>
</COL_A>

I have implemented IStateManager. It works fine for other items i have
in this control. But for this property its no able to save this values
and gives me a blank control while rendering.

I have applied ParseChildren and PersistChildren.. Also
DesignerSerializationVisibility(DesignerSerializationVisibility.Content),
PersistenceMode(PersistenceMode.Attribute)

So i am totally clue less why it will not save as the serialization is
happeneing properly. Do i need to implement a typeconverter?

Thanks
Rahul