Custom ringtone sounds good on computer, can't be heard on phone
Plug your phone into some large speakers.  If you can hear the tone much better, then the problem might be the small size of the speakers on the iPhone, which roll off low frequencies.

To prevent this, make sure any low pitched notes in your ringtone are made with timbres rich in strong higher overtones.

Added: If you create a ringtone from a stereo file, it's probably best if both channels contain exactly the same data.