Using measures in rolling 6 month averages

Hi,

I'm having trouble working out a rolling average

I have a measure I've created:

Sentiment PosNeu = table1[Positive+Neutral Sentiment] /table1[Positive+Neutral+Negative Sentiment]

This is a percentage calculated from sums of integers across 3 different values (so, Positive + Neutral Sentiment is calculated using sums of the 2 values)

I've then calculated a rolling 6 month average based on this:

Sentiment PosNeu R 6M Ave = CALCULATE(AVERAGE(table1[Sentiment PosNeu]),DATESBETWEEN(table1[startDate],DATEADD(LASTDATE(table1[startDate]),-6,MONTH),LASTDATE(table1[startDate])))

However, it's telling me that table1(sentiment PosNeu) either can't be found or can't be used in the expression - any ideas why this is the case? 

Many thanks in advance!

Chris