This happened to me. My left arm just couldn't lock out weights when pressing. I couldn't hold my upper arm in position when I tried to do extensions. My left arm kinda "drifted" out to the side when I bench pressed.
For whatever reason, I had trouble keeping my left shoulder tight and in place. It would kinda "wing out" on me. I started doing some kettlebell presses, holding the 'bell in the bottom b up position. This forced me to keep my forearm verticle, my shoulder tightly packed, and my wrist in-line. In just a few sessions, my shoulder felt much more stable and "activated."
Band pull-aparts, face pulls or upper back stuff on TRX handles (face pulls, scarecrows, inverted rows) are all good ways to warm up, and pull your shoulders back and down before you start real upper body stuff.