Stored Procedures Multiple EXECs

My stored procedure looks like this:

 ALTER PROCEDURE dbo.Calculate

AS

BEGIN

SET NOCOUNT ON

declare @lastUpdate varchar(15)

declare @sql nvarchar(4000)

declare LastUpdate cursor for select updateID from currentUpdate

OPEN lastUpdate

FETCH NEXT FROM LastUpdate INTO @lastUpdate

SELECT @sql = 'INSERT INTO A ...WHERE ..' + @lastUpdate

EXEC(@sql)

SELECT @sql = 'INSERT INTO B ...WHERE...' + @lastUpdate

EXEC(@sql)

SELECT @sql = 'UPDATE  ...WHERE...' + @lastUpdate

EXEC(@sql)

CLOSE LastUpdate

DEALLOCATE LastUpdate

END

 

When I run this stored procedure from my web page, only the first sql is executed and then I get a timeout message.

When I execute the stored procedure manually on the server, all the sqls are executed correctly.

How can I solve this problem?

Thank You.