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If the unit is connected by HDMI, note that this can cause some problems with some UHF channels for which the remedy is a different HDMI lead.
You can check if this is the cause by connecting via scart instead and then see if you can tune the missing channels.
Another possible cause is that the particular mux which contains the missing channels is just at a critical reception level and is falling below the receive-ability threshold of the DVDR.
You could try getting as direct an aerial route as possible to the DVDR with as few connectors as possible and leave the TV temporarily disconnected from the aerial system then scan for channels again.
This may give you channel placeholders for the missing channels [ If it does - do not rescan]
Reconnect as normal and then see if the reception is good enough on those channels. If not, you may require some additional boosting.