I do use the QC plugin and it is marvelous. But does not address series AFAIK.
I have clicked, resorted, converted, checked tweaks etc. The behavior is very consistent in its inconsistency
Chances are you have a series starting with The, A, or An. If you could be so kind as to click the 'T' book icon in the tag browser and scroll down the list if neccessary you will see all the ones with 'The' they all appear. But they won't appear in the expanded list.
I will try the manage series although I am sure I have done this already. Just restored my computer and have not reinstalled calibre yet, so may take a bit.
Thanks for the input
Helen