M-Audio FastTrack USB - very low signal level (line/guitar)

I've used FastTrack USB for a couple of years with a microphone through the XLR input with no problems. Now I am trying to connect a line output of a synthesizer to the line input of the FastTrack and the signal level is very low. There is no noise, it's just quiet, even with the track volume all the way up it's still no match for the rest of the tracks.
Any suggestions, except for trying all the knobs and switches on the face and back of the unit (tried that already)? Anybody has similar setup?
I use FastTrack for audio input, and built-in audio for output, it's connected through a hub (no problems for two years). In GB microphone is going to the Channel 1 (mono), and still works fine. Synthesizer goes to Channel 2 (mono) and is really quiet. I tried the syn with two different 1/4 cables with no difference. The headphones jack on the front of the FastTrack works fine with the normal level adjustment range, and if I plug in the headphones directly into the line out of my syn, I have sound, quiet (like you would get by connecting headphones to any line out, turntable even) but steady, so the syn must be fine.
Any thoughts?