SQL updates disappear.

Here is the code :
***************
CURSORSETPROP("Buffering",5,"mytable")
mid_fld=sys(2015)
begin transaction
Update mytable Set id_fld = mid_fld, flag1 = 3, adate3= date() ;
WHERE mytable .id= some criteria here
( usually 5 to 10 records are updated)

If ! Tableupdate(1, .T., "mytable")
=Messagebox("error in updating : mytable”, 16, oApp.Title)
Rollback
Return
ENDIF
End transaction
FLUSH
Select * from mytable where id_fld=mid_fld into cursor Cur2print.
********
It selects the records just updated as expected ALL THE TIMES and usually the
customer produces a hard copy of these transactions.

However in one case (out of the 30 installations of the application ) and in about one out of 20 updates the new values of the fields disappear as though they have never updated (as a proof the customer has a hard copy).

Visual FoxPro 8.0 Service Pack 1
WINDOWS 2000 SERVER
As far as I know the only difference from other installations is that there is hard disk mirroring.
Any explanation ?