Previous Period within SUMX Issue :(

Hi Community,

i just discover an issue in the calculation of sumx in previous period( previousMonth(), lastYear()..etc).

I explain the context: 

i have a sample table containing the columns: date, product, unit price, quantity

And i need to calculate the evolution of sales by month( divide Sales month by Sales month -1 ), so logically i will use the following formulas: 

Sales Amount = unit price * Quantity

and to do it correctly i used sumx like: 

Sales Amount = CALCULATE(SUMX(Sheet1; Sheet1[Quantity]*Sheet1[Unit Price]))

then i calculate the sales of the previous month like: 

Previous Monthe Sales = CALCULATE([Sales Amount]; PREVIOUSMONTH(Sheet1[Date]))

then i used an other measure to calculate the evolution like: 

Sales Evolution = DIVIDE([Sales Amount]; [Previous Monthe Sales])

the result i got was blank which is not logically, for information i tried also to get the measures in a summaized table and return the calculation but it didnt work

What you can suggest please, any one have an idea.

 

Thanks in advance