Where you set the sring tension is everything. take the straddle off, and tighten both arms on, ,when they're about 1/2 way open. Straddle up, and see which oneis closer to the rim. This one has LOW tension, and it needs improving. Undo straddle and the arms should return to where they were. The one that needs more spring tension, loosen the bolt, open it 1/4" more than the other, bolt back up and see where you go. When nearly there, you should be able to trim with the allen key, BUT, you MUST remove tension from the spring (i.e. open the brake again) BEFORE you adjust, or you'll split the platic agjuster plates in the housings.
hope this helps,