Cube With Monthly Data Chunks Not Working

I have a table based around requisitions, and each requisition has a number of positions. That number can change over time through updates to pertinent rows rather than through transaction-like records that record an entire history, and I'm only able to get a monthly snapshot of the table. What I decided to do is still use one table for OLAP (fact_requisitions) but add a column called period_key that refers to the month the data comes from. So if I have two months of data then the table has each requisition twice, possibly with differing position counts, and new requisitions from the second month are only present once. Then I tried to filter the MDX query like so:

SELECT {
([Dim TimeRequestClosed].[Year - MonthNumber].[Year_Text].&[2008].&[1],[Dim Requisitions].[Period].[Period Key].&[200801])
}
ON COLUMNS,
NON EMPTY
{
([Dim Location].[Region Name].MEMBERS, [Dim Location].[Period Key].&[200801])
}
ON ROWS
FROM
[Requisitions]
WHERE
[Measures].[Request Closed Date Count]


This query doesn't work even though the data is there, it just returns nulls. Am I going about this all wrong? If not, what might I be doing wrong, and how would I get the query to return more than one period (e.g. tell Dim Requisition to match up with Dim Location on the period key)?