When entering a QSO in the "New QSO" window if the call has been worked before then previous QSOs normally appear in the sub-window at the top of the dialog.

If the previous call has a national prefix then it is not matched. The example I found was that I worked OK/DL3SBA/P and another day I worked DL3SBA/P When I logged the second QSO the first was not shown.