What sort of external deck is it? Record deck, Cassette deck, Reel to reel etc???
If it's a record deck it might need to go through an amplifier first.
What socket on the deck are you taking the feed from. On my cassette deck which doesn't have a line out I use the speaker output. Works fine.
Also, silly question, but are you sure you are plugged in to the line in and not the mic socket?