Have someone check your form to make sure both your elbows are following the same pathway. If you're less than a year of hard training, you likely just need to fill in more. You will always notice issues with symmetry early on in the game. The bigger you get the more it disappears.
Unless of course you tore your AC joint, displacing your clavical 1" forward and taking your anterior delt along for the ride, thus placing it into a position that competes with your upper pec region on all pressing movements. But even with this being my case, my left upper pec is only slightly under developed. Few people notice unless I point it out.