|Remember these? An early digital airband radio from Swinburne|
This situation frequently occurs for me with Swanwick military central because I can always hear the controller on 252.875 UHF but if an aircraft is talking to him on his VHF frequency (128.7) I usually only hear the aircraft side of the conversation (probably different transmitter sites in use).
Again, it can be down to a lot of factors such as your location and the location of any actual transmitter sites that 'repeat' the transmission from Swanwick etc.
But hopefully, that gives you an idea, I'm no radio 'tecchy' so would not even try to explain the more technical aspects of this!