Concurrency violation problem

[Agnes]

i got an invoice table, its primary key is recid, I set it in SQL server 's
table, (set identity (yes),indentity seed (1),indentity increment(1))
Step(1)Now, in my "btnNew" button,
bmInvoice.endcurrentedit(),bmInvoice.addnew(),
in my"btnSave",
bmInvoice.endcurrentedit()
if dsInvoice.haschanges() then
daInvoice.update(dsInvoice,"invoicetable")
dsInvoice.accpetchanges
end if

Step(2) in my 'btnMoidfy" , bmInvoice.endcurrentedit(), then I process
'btnSave' again,
Problem , I found that After Step(1), the primary key recid got nothing in
it. IF I 'modify' the record and perform 'btnSave' again, I will got
'CONCURRENCY VIOLTATION, UPDATE COMMAND 0 RECORDS',
If I quit the form, run it again, I can Modify and save my record, What's
wrong ???
please help ~~