Weighted Average Calculation

You need to provide a Row Context for this to work when there is more than one Group selected. 

Try using: 

Weighted % = 
VAR FR = 
DIVIDE(sum([Risk2]), calculate(COUNT([Risk2]),ALLSELECTED([ITEMCODE])))
RETURN
SUMX(VALUES([GROUP]), 
SWITCH([Group]
, "A",FR * 0.5 
, "B", FR * 0.3
, "C", FR * 0.2
, "D", FR * 0
, "E", FR * 0
,"F", FR * 0
,FR * 0
)
)

Also, you can probably use the AVERAGE function instead of DIVIDE, SUM and COUNT.