vlookup error with subtotal
 

I have 4 worksheets in 1 spreadsheet. The 1st worksheet contains a summary
of subtotaled information on the other 3 sheets.

I subtotal sheets 2-4 after sorting the data by the date column.

In sheet 1 I use the vlookup function to capture the subtotaled fields based
on the date.

Here's the vlookup function:
=IF(ISERROR(MATCH($A14,Match_CDA_Date,1)),
0,
VLOOKUP($A14,CDA_Weekly_Totals,10))

This function works fine for weeks 1-8; after that I either get a zero or
erroneous data returned. This error occcurs in different cells for each
worksheet.
The error may start in row 10 for sheet 2, row 13 for sheet 3, and row 9 for
sheet 4.

Why does the vlookup work okay in some cells and fails in rows further down?