search for missing matches

Let's test this...

Select C1 in Leadsheet and name it InputColumn.

Select A1:AA15 and name it DATA

Then select C2:G14 in Month1! sheet.. and apply conditional formatting with formula: =AND(C2>0,C$1=InputColumn,ISNA(INDEX(DATA,MATCH($A2,INDEX(DATA,0,1),0),3)<>C2))

does this do what you expected? The red 3 is the column number in DATA that corresponds to month of interest.