When you retune the DMR-HW120, it will tune in all the channels it can find automatically.
If some channels you were previously getting are now missing, then your signal strength may not be as good as it was previously.
If you aren't already using a signal booster, you can try adding one to your aerial. These are widely available from many stores, including Currys and Argos: http://www.argos.co.uk/static/Product/partNumber/1068388.htm
If you are using an external aerial, you should also check that it hasn't moved slightly; it may still work for other TVs connected to the same aerial, but the loss of signal may be just enough for the DMR-HW120 to not pick up your missing channels.
Let me know if I can be of any further assistance.Best,Pete.
Thanks, ***** ***** appreciate that. As far as I can see, my external aerial has not shifted, and why should I lose channels when I retune when I had them all before retuning?