Bunch of possible causes:
1) Crappy pads. Stock Tektro pads, for instance, tend to suck. Kool stops are much better.
2) Worn pads.
3) Gunky rims/pads - cleaning the rims and pads helps, many times.
4) Badly aligned pads - if you have a lot of toe-in, or if the angle's off top to bottom, you might not have a lot of brake surface actually touching the rim.
5) Lube on pads/rims - get a little lube on there and good luck trying to stop the bike
6) Not enough travel - it's possible you think it's locked but the lever's actually reached the end of its travel
If it's worth it to you, one test to do is to put the front pads on the rear rims. That should eliminate one variable, anyway.