I have tried 1 and 11, similar result. I just gave 6 a go but again, no luck.Have you tried selecting some other channel besides an edge channel? Channel 1 and 11, at least here in the US, are considered edge channels and its harder to get main+aux channel (HT40) working correctly when an edge channel is selected.
The funny thing is that nothing much has changed in the neighbourhood and it used to work. In the last few weeks it just stopped and I can get it to come back to two channels.