SQL Trigger problems

We use a program called Fedex ShipManager. It integrates with a SQL 2008 database. I am trying to write a trigger that will copy some of what the fedex software writes to a different database. I have checked, there is no way to get the software to write to two separate databases. This trigger works when I insert the data, but it fails when the fedex software does it. Does anyone have any guesses as to why this would happen?

ALTER TRIGGER [dbo].[trg_FulfilmentRequest]
   ON  [Billing].[dbo].[ShipmentBilling]
   AFTER INSERT
AS 
BEGIN
	-- SET NOCOUNT ON added to prevent extra result sets from
	-- interfering with SELECT statements.
	SET NOCOUNT ON;

	--Check if the ReferenceCode is numberic
	
	DECLARE @ShipRequest varchar(50)
	SELECT @ShipRequest = ReferenceCode FROM INSERTED
	IF ISNUMERIC(@ShipRequest) = 1
	Begin
		--If the Reference Code is numberic convert it to an integer
		DECLARE @ShipRequestID INT
		SET @ShipRequestID = CONVERT(INT, @ShipRequest)

		--If the ReferenceCode is in CatalogData.dbo.ShipmentRequest
		IF EXISTS (SELECT * FROM [CatalogData].[dbo].[ShipmentRequest] WHERE [CatalogData].[dbo].[ShipmentRequest].[ShipmentRequestID] = @ShipRequestID)
		BEGIN
		 --Insert the data into CatalogData.dbo.ShippingInformation
			DECLARE @TrackingNum varchar(50)
			SELECT @TrackingNum = TrackingNumber FROM INSERTED
			INSERT INTO [CatalogData].[dbo].[ShippingInformation] (ShipmentRequestId, TrackingNumber)
			VALUES (@ShipRequestID, @TrackingNum);
		END
	END
END

I have tried commenting out various lines. It is the insert statement that causes the problem. It also causes the problem if the TrackingNumber is removed.

Thanks for any suggestions