It depends on what the value you are pivoting upon is. If it is very simple, such as a month number ranging from 1 to 12, you can define static column headers for the pivot field (by going to the pivot field's properties in design view and setting the Column Headings property to a list of all possible headings, like this:
1, 2, 3, 4, 5, 6, 7, 8 , 9, 10, 11, 12
If you use a more complex column heading for the pivot you will need to generate all possible values for that column heading in a separate query, and then left-join those values to the crosstab so that there will always be a row for the particular month you need.
Unfortunately, Access cannot lookup pivot headings dynamically in response to changing circumstances - a weakness with crosstab queries.
There is a slower method to get what you want without using a crosstab at all, but to advise further on this method (which uses a series of manually-defined fields for each month to SUM the relevant value only if a particular month condition is true) I'd need to see the current SQL for the crosstab you are using at present.
-Stewart