Hi
On your remote
Menu-setup-service searching-sat config
Now you should see your list of sats the end option controls LNB polarisation just make sure all are set to H/V and not 14v or 18v.
If not as a test!
Put your 500 in a loop from your 600 so your 500 is last in chain. With the 600 switched to a horizontal sat see if you can see the same channel your 600 is on.
If you then get horizontal channels its possible tuner has power fault.
You could also use a multimeter to test voltage while on horizontal frequency
Ferret