I am Pete, and I'll be happy to assist with your question.
Check if you can view all channels.
To rectify this, try unplugging the satellite cable from one of the tuners.
* sorry some of my previous text is missing due to a website error *
I will start again:
The DMR-BS750EB has twin tuners.
If one of them has a bad connection or if that tuner has failed, then it will be unable to record whilst another channel is being viewed or recorded at the same time.
Then check if you can view all channels.
Now disconnect the cable from the tuner you just checked and connect the second cable to the second tuner.
Check again if you can view all channels.
If you are unable to view channels when only one of the tuners is connected, then that tuner is suspect.
Test it by connecting the other cable to it.
If it still won't work then the tuner is faulty.
If it does work when the other cable is connected, the one of the cables is bad.
Check that both cables are securely connected to the LNB on the satellite dish.
Also check the connector wiring on the cable terminations.
Do let me know if I can be of any further assistance at all!
Thanks for your prompt response.
I have tries your suggestions and all channels work perfectly.
I am sure that I'm not able to give you a description of the problem in terms that would enable you to provide a solution.
It would need a visit by a technician to view the all connections and settings before a suitable solution is found.
Thank you for your help.