Probably because the connections to the back of the wall plate are directional, and there are frequency filters, and you probably haven't got direct connections in the way that would make it possible to stuff the satellite LNB signal in to any old hole and make it come out right.
From the sound of your description you've got the satellite LNB signal lead(s) coming in to the room right next to the telly virtually. So why the hell do you think you need to put them in to a socket on the wall only to have them travel half way round the house before coming out of another socket an inch or less away and to then have another cable from the wall going to the back of the TV?
Just plug the damned signal in to the back of the telly and have done with it... lol
This is how your Labgear should be wired ONLY IF your satellite LNB signals go
directly in to the loft. But yours don't.
In the diagram above I have arrows marking the directionality of the signal, and I have used colour coding to indicate frequency ranges.
Directionality is important. Signals flow in a direction from a source to sink device (sink device = a recorder, a tuner, a TV display, an amplifier etc). So just like traffic has a flow, and you wouldn't (I hope) consider driving the wrong way down one side of a motorway, then with signals you mustn't feed a signal in to a socket or up a cable that has a signal in the opposite flow.
Look at the diagram. Both the satellite outputs on the wall plate are "
traffic flowing towards you " sockets. These are outputs. When you have the cable in your hand from the LNB then the signal in that is flowing towards you too. Connect the cable to the socket as above and you'll have two signals heading towards each other - A big crash.
Frequency is important. Look at the diagram again. The only socket on the wall plate designed to take a signal back up to the amp is the Return socket. This is a "
traffic flowing away from you " socket. It's an input. However, it can only take a certain size of traffic flow. The size is dictated by the frequency. Lower frequencies are the equivalent of smaller vehicles on the road. For this example, FM / DAB / TV signals are the equivalent of motorbikes, cars and small vans. Satellite signals would be like HGVs. Now imagine this road has a low bridge. Cars, bikes and small vans can get under, but HGVs would crash in to the bridge. That's how the Return socket works. It will let FM / DAB / TV signals pass, but will block Satellite LNB signals.