vb中Update 语句有时不成功

在VB6+SQL SERVER2000中:
修改数据有时不成功:
strsql = "Update tblTempDia set Dia=" & Dia & ",UpdateID='" & sUserID& "',UpdateTime=getdate() " & _
" where PrinterDate='" & ADate & "' and NewPaperID='" & sNewPaper & "' and PaperMac='" & sPaperMac & "'"
Cn.Execute strsql, iAffect*******iAffect有时少于等于0,这主要是什么原因引起的???