Unable To Adjust Mechanical Disc Brakes Properly
Mechanical disks almost always operate one side only. Setting them up correctly is not hard but needs care. 

Refer http://www.parktool.com/blog/repair-help/avid-mechanical-disc-adjustment#article-section-3 for detailed instructions for Avids - technique should work for other brands. 

Essentially, slack of the cable and caliper mounting bolts,  Undo both pads (counter clockwise) then tighten the outer pad 1/2 turn. Tighten the inner (non-cable side pad) until the disk is clamped tight and then tighten caliper bolts to set the caliper. Tighten the cable, loosen the inner pad 1/2 turn and outer 1/4 turn, then adjust as needed for modulation. 

Although this sounds difficult and long winded, its actually only takes a few minutes once you get used to it.