Modules & VBA :: Syncing Combo Boxes Which Are Also Lookup Values

I have two tables,

tbl_Retainer
tbl_Retainer_Grant_Funding
tbl_Retainer has the field,
Retainer_ID

And tbl_Retainer_Grant_Funding has the fields,
Retainer_ID (a lookup field from tbl_Retainer)
Agreement_Num (a lookup field from tbl_Grant)

I have a form based off of a query(not sure if that matters), that uses that tables, tbl_Assignment and tbl_Assignment_Grant_Funding. These tables have the above fields as lookup fields.

So...what happens is, if an Assignment has a Retainer, I want the Agreement_Num box to show only the Agreement_Num's associated with that Retainer, otherwise just show all the Agreement Num's.

In my form, I have Retainer_ID with the row source,
SELECT tbl_RETAINER.Retainer_ID FROM tbl_RETAINER;

And Agreement_Num with the row source,
SELECT [tbl_GRANT].Grant_ID, [tbl_GRANT].Agreement_Num FROM tbl_GRANT ORDER BY [Agreement_Num];

In my AfterUpdate event for Retainer_ID I have,

Private Sub Retainer_ID_AfterUpdate()
Dim strSql As String
strSql = "SELECT [Retainer_ID]," & _
"[Agreement_Num]," & _
"FROM tbl_RETAINER_GRANT_FUNDING" & _
"WHERE [Retainer_ID] = " & Me.Retainer_ID.Value

Me.Agreement_Num.RowSource = strSql
Me.Agreement_Num.Requery
End Sub

When I am in my form and choose a Retainer ID, the Agreement_Num box goes blank, and there are no choices to choose from. I am wondering if this is because the Agreement_Num's are sourced from tbl_Grant and not from tbl_Retainer_Grant_Funding.