触发器数据遗漏 急

CREATE trigger MsgUpdate_Trig

on dbo.MSG_Inbox

for update

as

    begin

        INSERT dbo.UT_Msg(Sender,MsgType,MsgTitle,MMSUrl,MMSContentLocation,MsgArrivedTime,MMSDownloadedTime,MMSDownloadedState,CommPort) 

        SELECT Sender,MsgType,MsgTitle,MMSUrl,MMSContentLocation,MsgArrivedTime,MMSDownloadedTime,MMSDownloadedState,CommPort FROM inserted 

        where MMSDownloadedTime is not null and MMSDownloadedState=100

    end

平时都没有见过数据遗漏的情况，但是昨天发现有数据更新了
但却没有插入到表UT_Msg中
怎么回事呀？
什么情况下才会出现触发器触发遗漏？