The environment could be absorbing the bass. treble sounds cut through the air more whereas bass apparently get's absorbed by structures more, therefore if there was say a wall in front of you the treble wouldn't go through it as it doesn't travel through structures well, but the bass would vibrate the wall and cause sound on the other side. bass sounds therefore might be absorbed by every physical object around you.

actually that might be too elaborate, how goods your bass amp? remember bass amps have to be MUCH higher wattage then guitar amps and if you wanna hear em you gotta crank em.

Then again maybe it's a lack of stage monitors.