Target it with isolation movements on exercises like dumbbell flies and dumbbell presses. So do those two exercises on the left side ONLY and do not do them on the right side.
Do 4 sets of 10,8,6,15 reps with 6 reps being the most you can lift in 6 reps, so its essentially a failure set.
* 10 reps you use 50% your 6 rep max,
* 8 reps you go with 75% the 6 rep max,
* 6 reps is 100% you can handle for 6 reps NOT your 1RM.
* 15 reps & 35% your 6 rep max.
Do that on each with GOOD form and not with jerking movements, your lifting the
weight not throwing it and in about 4-6 weeks you should see the left side come out more.
This is a good technique to bring lagging sides up. The 10,8,6,15 method is actually created by the Great late Vince Gironda. It WORKS my friend and you can use this on anything.