Thank ou for your help. As data was sensitive, I recreated the scenarion using test data, and it worked!
So this made me look elsewhere for the problem.
I had added data to the table manually for a new Month, and had left some of the fields blank (ie ColA and ColC were blank).
That is why the New Month did not load.
Once I entered data (or 0, but not '-'), everything was ok?
If you can advise what the rules are about blanks (or '-', that would be great)? Is there something I can do in the load to ignore the situation where there are blanks ie to treat as 0?
Anyway, thanks for your help.