Can only get a good pump in one arm

Hey Marc, no matter what I do, I can not get a pump in my left arm. I use dumbbells for things like curls and bench, and focus on my form to make sure I'm not favoring one side. Both arms reach failure at the same amount of reps so I know I've evened out the strength imbalance I had from a previous hand injury. However, while I feel an AMAZING pump in the right arm, the left barely feels pumped at all by the time I'm done with my post workout shake. What is the issue, and what can I do to feel that amazing swoleness on bother sides?