Ok, SOLVED!
Here's what I did:
On related form instead of using: DoCmd.OpenForm "frmWorkInstruction", , , "fldWISerial= '" & Me.fldWISerial & "'"
Instead I used fldDocID to locate record, like:
DoCmd.OpenForm "frmWorkInstruction", , , "fldDocID= " & Me.fldDocID
Thanks everyone for help.