I think what's happening is expected behavior due to Data Blending. I'm guessing you've got the blend set up to be linked on the Month column. Since the primary data source only has data up through November, the blended data source will only bring in rows that match Month names between each data source. Since December isn't in the primary data source, the secondary data source's value for December won't show up.
To work around this you could create a separate Date data source that is set up like this:
February March April May June July August September October November December
You would then have this date table as the primary data source, and blend both the Measure 1 and Measure 2 tables with the linking field as Month. You could then bring Month from DateTable out onto the viz, and then both of the measure columns from your Measure 1 and Measure 2 tables onto the viz. This will give you marks for all months.
Hope that helps!