Calculate Beginning Balance to Roll forward from December Prior Year

I'm trying to calculate a Beginning Balance account to Rollforward from the December Prior Year Ending balance account. I can't seem to get it to work. This is the latest version of the calculation that I'm trying:

SET HYBRIDBSOINCALCSCRIPT NONE;

FIX ("Entity Currency", "FCCS_Entity Input", @Relative("Intercompany",0), @Relative("Total C1", 0),@Relative("Total C2", 0) @Relative("Total C3", 0), "FCCS_Periodic" , "Ovr" , "FCCS_Managed Data") "GrsUnrealGLFixedMatEOP" ( @CALCMODE(BOTTOMUP);

IF(@ISMBR("Jan")) [GrsUnrealGLFixedMatEOP] = @PRIOR("Dec"->[GrsUnrealGLSubOthEOP]-> @Relative("Entity",0), 1, @RELATIVE("Years",0));

ELSE [GrsUnrealGLFixedMatEOP] = @PRIOR([GrsUnrealGLSubOthEOP]);

ENDIF

ENDFIX

This code worked in the member formula. However, when I tried to add it to insertion rule 10, it did not give us consistent results. Sometimes, the calcs worked and the other time, they did not work.