Service tax simulation in service entry

Dear all

In a particular PO created on 01.11.2011 service tax condition was given. Service entry was done on 04.11.2011.Later on 09.11.2011 service tax condition was changed to zero tax rate.Now when the user is doing MIRO on 10.11.2011,still service tax portion is coming under taxes portion.

Can any one explain me what could be the reason.

Thanks in advance.

Madhavi