How do you construct comboboxes?
Do you assign it to the cell with QTableWidget ::setCellWidget?
If yes these combo boxes are completely independent widgets unrelated to QTableWidget.
So no itemChanged or any other table related signal is going to be sent when you manipulate with cell widget (unless you do it yourself).
In reality using cell widgets screw your table view navigation and selection.
I would highly recommend to avoid cell widget approach and leave it only for very simple cases.
In your case it probably would be better to use column specific delegates.
Recommendations from the top of my head:
find is a Spin Box Delegate example in documentation.
Make your own subclass similar to in example.
set autoFillBackground(true) to your editor widget in the createEditor method.
Set openPersistentEditor on your view's for desired rows/columns.