Event stops being invoked after several seconds

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C#

Office

MS-Excel

, +

I am writing a Diploma Thesis with a group

To manipulate a Cell within an excel sheet we use the Excel.DocEvents_SelectionChangeEventHandler

public Excel.DocEvents_SelectionChangeEventHandler AssignSelectionChanged;

[...]

AssignSelectionChanged = 
  new Excel.DocEvents_SelectionChangeEventHandler(SelectionAssignChange);
_XlBook.Worksheets[1].SelectionChange += AssignSelectionChanged;

For the first few seconds everything works perfectly and our SelectionAssignChange method is being called via the event.

private void SelectionAssignChange(Excel.Range target)
{
   [...]
}

But after some time (differs from try to try) the program just ignores this method and no further cells with dropdowns are created.

Imagine it like
Dropdown Enter
Dropdown Enter
Dropdown Enter
Dropdown Enter
Dropdown Enter
5 seconds passed
Nothing Enter
Nothing Enter
Nothing Enter

Hope any of you can help me!

Cheers