Some things to try (in order)
1)Turn up the bass on the ipod's equalizer
2)Ensure connections are adequate
3)Listen to a song with lots of bass so you are sure to hear it
4)Turn up the sub
5)Turn up the bass on the receiver's EQ
6)Try putting it in stereo mode
7) make sure the sub is enabled on the receiver when you are on a certain input or listening mode

Lastly
8)Get an ipod dock for it (they sound better than 1/4" to RCA anyway)