@@Rowcount Or Output Parameters Error

I have the following stored procedure working with an Access 2000 front end. The output parameters returned to Access are both Null when the record is successfully updated (ie when @@Rowcount = 1), but the correct parameters are returned when the update fails. I'm a bit new to using output parameters, but I have them working perfectly with an insert sproc, and they look basically the same. What bonehead error have I made here? The fact that the record is updated indicates to me that the Commit Trans line is being executed, so why aren't the 2 output parameters set?

TIA

EDIT: Solved, sort of. I found that dropping the "@ResNum +" from "@ResNum + ' Updated'" resolved the problem (@ResNum is an input parameter). This implies that the variable lost its value between the SQL statement and the If/Then, since the SQL correctly updates only the appropriate record from the WHERE clause. Is this supposed to happen? I looked in BOL, and if it's addressed there I missed it.

CREATE PROCEDURE [procResUpdate]

Various input parameters here,

@RetCode as int Output, @RetResNum as nvarchar(15) Output

AS

Declare @RowCounter int

Begin Tran

UPDATE tblReservations
SET Various set statements here, LastModified = @LastModified + 1
WHERE ResNum = @ResNum AND LastModified = @LastModified

SELECT @RowCounter = @@ROWCOUNT

If @RowCounter = 1
Begin
Commit Tran
Select @RetCode = 1
Select @RetResNum = @ResNum + ' Updated'
End
Else
Begin
Rollback Tran
Select @RetCode = 0
Select @RetResNum = 'Update Failed'
End
GO