Series Labels in Numbers Charts

I'm having a problem putting a series name label onto my bar chart.

The legend for my two series is underneigth the chart.

For the 1st series I was able to click the text that was automatically povided and change the series name to the right one.

For the second series though, even though there is a coloured square identifying it, there was no text made for it and I can't even add text by clicking the legend, or by using the chart inspector.

When I use the chart inspector, type in the series label, then tick it, the text just dissappears and no label is added.

Does anyone know what I'm doing wrong?

I'm using Numbers 2.2 (156)

Thanks