A guess would be that the second tuner connection is faulty. Check the f connector is tightly screwed on the coax cable.
You don’t say which box. Assuming it’s a second generation box and you have an english postcode.
Try this test. While watching BBC1-HD press the instant record button. Then use the channel up/down button. That should cycle through all available channels.
If you can only get these
102 BBC Two HD
106 BBC One HD
600 CBBC HD
971 BBC Two HD Wales
972 BBC One HD
949 BBC Two HD
979 BBC Two Wales
978 BBC One NI HD
Then tuner 2 has no signal.
Also try pressing the home button and select setting System Information – Signal Info. What Does the two tuner bar chart say and what does it say at the bottom of the screen Freesat box connected with ?????