Cumulative Sum Not Working

Hi all.

First time post, so please forgive me if the answer is obvious. I'm seeking to create a pareto chart with customer name on the x axis and volume on the y. This chart must be dynamic and adapt when I change the date fields and selected site. 

I have this working perfectly when only one whole month is selected by first building a new table as follows:

MMByCustomer =
SUMMARIZECOLUMNS (
'Query1'[site_name],
'Query1'[customer_name],
'Query1'[full_date],
"Result",sum(Query1[actuals_material_margin_corp])
)

And then using the following measure:

Cumulative2 = SUMX (
FILTER (
ALLSELECTED (MMByCustomer),
'MMByCustomer'[Result] >= Max ( MMByCustomer[Result])
),
MMByCustomer[Result])

The result for one month is perfect:

Unfortunately, when I expand to two months, the cumulative total stops functioning as expected:

For the record, when I filter, I am using slicers linked to MMByCustomer[full_date] and MMByCustomer[site_name].

Many thanks in advance,

ConfusedWolf