Low EBNO on T10

My guesses:

LNB connector problems - try wiggling the connectors and see if that causes variations in your receive Eb/No. Corrosion of the centre wire pin and the braid is most likely cause. Also pin not long enough, braid not making good contact or loose braid wire fragment.  If the pin is burned at the LNB entry then rework the cable termination but also get a clean length of centre wire and push it in and out of the LNB to clean the spring contacts inside the LNB, which may be corroded also, unfortunately.  A smear of electronic grade silicone grease is recommended as this seals the exact point of contact of the two metals.  The joint must be wrapped externally to exclude moisture.

Water in the LNB input waveguide path. More likely if the LNB and its transmit reject filter are underneath the feed system.  Water in the rings inside the conical feed horn.  The water collects in the lower ring slots only and will affect one polarisation more than the other. Also large spider or insects inside.

Cross pol interference.  A new carrier may have come up on the opposite polarisation and may be interfering with you, either because your polarisation setting is poor or because the other transmit site has poor adjustment.

WiMAX wireless interference from new nearby terrestrial mast operating at the lower end of C band.  If your receive frequency is above 3.7 GHz and the interference below this then it may help to use an old 3.7-4.2 GHz LNB rather than a modern wideband 3.4 - 4.2 GHz LNB.

LNB fault.  Does fault appear or disappear with temperature change (dawn and dusk ?).

Best regards, Eric.