I did everything you advised and a lot more, as I switched all the settings again and again.
I also used a plug-in "diseqc tester" that is available to download. That showed something strange:
Diseqc commands work perfect for Astra, but not for Hotbird.
When Voltage mode is set to polarization the box doesn't get any services from Hotbird.
When Voltage mode is set to 13V all Horizontal Polarization services can be tuned.
When Voltage mode is set to 18V only Vertical Polarization services are tuned.

The thing is that I cannot get both Horizontal and Vertical polarization services at the same time...
(Problem remains only for Hotbird. Astra works fine)

Hope someone could help with that...

Tried it...
Still getting less than half of the channels from hotbird, Astra seems to be working fine.
I also tried the "increase voltage" set to "yes" with no luck.
Please let me know if you have any news.

p.s. Channels are working if I connect another box, but not DM800

thanks M8!
I will try that right now!
Let you know what happens!

I have the same problem with post No 8 with my DM800, Is there any solution for it?

what settings should I use in order to receive both 13.0E and 19.2E in my DM800.

My configuration is like that:
single dish with two "MTI" universal lnbs
A/B DiSEqC from "MAXIMUM"
and "DM800".

I have tried many different combinations but not managed to get both sats to work ta the same time.
I cannot even tune my box to reveive all channels from the one sat.