i had the same problem...i would train my bi's and tri's together and got some nice results by training w/ maximal intensity w/ approx 8 sets each in the 6-12 rep range(pyramiding).i experienced a world of difference though when i added some additional volume to this.now i train 16 sets for bi's and 16 sets for tri's and have gained way more size...i know it sounds extreme,but it's working for me and perhaps it can work for you.the point to my story:add volume and wake those fuckers up!i used to beleive that less is more as practically everyone advocates today...but in my case,more is better.learn to read your body and remember that if something is lagging,it's for a reason!!!fyi