It could sometimes be that specific values from your meter, or sometimes even all values, is missing in the modbus map.
Since the modbus map is directly connected to the measurementseries (which is an editable clone of the raw data values from the meter) it is most likely that the main problem is, that there has not been any measurementseries created.
The measurementseries normally get created when you install the meters the first time, or when you manually press read the meter buttons in the UI.
The rules that will come into action and dictate what is going to be created as a measurementserie or not is located under measurementseries->templates
The templates defines how the measurementserie will represent the real value, and which values.
So, if there is values missing in the modbus map and in the measurementseries it is probalby because the meter you have isnt included in the tempates.
The quickest way to fix this is to simply inactivate all templates, then active "other devices". This will allow ALL data to become a measurementserie.
After this you simply need to enter the meterlist and read/store value on the specific meter by pressing the floppydisc icon (or "read all & store" for all meters if needed).
And it will generate the measurementserie and add it into your modbus mapping. When this is done you can undo the changes you made with the templates mentioned above.
note that this will allow ALL available data to become a measurementserie. If you dont want all that information, just delete the one you dont want to keep.