Use of Substitution variable in Custom Calc in EPCM

Summary:

Substitution Variables used in Custom Calcs are not working though rule shows as Valid.

Content (required):

We have a use case where we need to calculate the Current month Balances after allocation. To do so we need to subtract the current month allocated values from Previous month.(as data is YTD) To achieve the same we have written a Custom calc.

 

It works fine, but the Problem is we don't want to hardcode the Month in calc as that would change every month.

 

We thought of using Substitution variables in the Custom calc. The application allows us to do the same and the rule is valid, but it doesn't work and after running rule it says 0 cells updated as warning.