When change combobox-observablecolletion does not refresh

Question

I have Combobox A and B. When change Cmb A, and then select Cmb B and then go To Cmb A change selection, come to Cmb B..

It remembers position of Cmb B it had last time..and by default scrollbar shows there..

Using Databinding with Itemssource and viewmodel..try collectionchanged event and set cmb B ..selectionIndex to -1 etc..But still same result.