Correct Value for Rows not matching KPI

[thefish]

I am designing a control table for my company and coming acorss an odd issue. 

The table has the following structure: 

1 dimension (we'll call it [Group])

A couple of measures most of which are master measures and 1 new measure (Days out of control)

The  days out of control measure works fine if not in the table and is in a KPI (It shows the number of days beneath the LCL)

However, when we move it into the table, I hit an odd error. the value of this days out of control is an error. 

The expression is listed below (modified to hide company data) 

SUM(AGGR(IF($(vValue)<$(vLowerLimit),1,0), [Date]))

I have tried fixing my =  signs in variables to find the solution but to no avail. 

Please help! 

Thanks in advance,

Fish