Unable to do more nested dropdowns in Top Nav

I know how to edit links in top navigation and do drag-drop to create nested dropdowns - see attached image.  I've already done one for our office locations, but when I added "Customer Service" and needed to add it as a dropdown under "Sales", it would no longer let me - the Customer Service link just slides to either side of Sales but refuses to go underneath.  I was able to add it as a dropdown within the existing dropdown for Locations, but it won't let me add it under Sales.  Also, it shouldn't matter but Customer Service IS a subsite of Sales.  I've tried making the change in Edge, IE and Chrome, to no avail.

Any ideas?  Is there a limitation I'm hitting?