Well there could be a few factors in play here. I would suggest doing some unilateral pressing as your secondary and assistance movements. Try some of the following....
-Half Kneeling 1 Arm Presses with either KB or DB
-Staggered Stance Standing 1 Arm Press
(just press overhead instead of horizontally and use a KB or DB)
These are my favorites but you can mess with these and see what works best for you. Ben Bruno has some good articles on the subject of unilateral pressing and he even uses a Landmine press with half kneeling and staggered stances.
I would do your stronger arm first and say you get 11 reps. On your weaker arm you would go until basically failure and lets say you get 7, rest 30s and then rep out again and say you get 3. Keep doing that until you get the same number of reps or more than your stronger arm. you could also just do your weaker arm first and then just match what your weaker arm can do with your stronger arm but I feel that the former style gives you some more volume and intensity needed to bring up this weakness.