Update the record succesfully ,after 1 hr, the record return to original"

PC1)user input invoice A-F in the morning, around 1:00p.m she changed the invoice no from A into 21, B into 22, etc. (print it all),she didn't quit the application .After 1 hrs, she can't Invoice21, I browse the table and find that invoice21 didn't exist , but invoiceA exist. Invoice 22,23 got no problem .It is very strange..
Of course ,there is another user use the system and input the invoice. (they may do tablerevert, or tableupdate.

in these two day, they got fatal error... and the record can't append correctly
I delete the *.cdx and remove the table from database, I create the index again .
It seems run normally, However, tableupdate 's problem still happend.

P.S- I put the table in the dataenvionment, the buffer mode is 5