Did you place them via Gruss?
If so you can check the diagnostic log file which will show a record of what occurred in Gruss, which might give you some clues if they didn't get placed correctly.
You can find them in Options/Open Settings Folder/Diagnostic logs there should be one saved for each day.
There is also the transaction log but that only saves if you check the settings to save automatically on exit. If you have not done that and shutdown Gruss in the interim then that record will be gone.
Did you check your lapsed, cancelled and voided bets records from the bet history page on the Betfair website?
https://myaccount.betfair.com/activity/bettinghistory