使用WiX创建第二个Service Pack

How do you use WiX to create a second service pack for your app? I’ve released SP1, and that installer works fine. But I’m having some difficulty with SP2. My current WiX setup upgrades from the original (RTM) version correctly. But if I try to upgrade from SP1, the files simply don’t update, although it shows as SP2 in Add/Remove Programs. What do I need to do to make the upgrade work? I’ve added another How do you use WiX to create a second service p