Sources of the scapulothoracic irregularity can include vertebral and rib subluxation which irrititate the nerves serving the muscles around and under the scapula causing spasms and a washboard effect between the scapula and ribs. Further, inflammation of the subscapular bursa can create poor articulation or scapulothoracic movement. Muscle imbalances and weaknesses in the trapezoidal, rhomboid, and SITS muscles can provide improper support for the scapula in motion or in weight resistance situations. RICE and insuring you are strengthening the SITs muscles as well as the traps and rhomboids can assist with this condition. See the fitness center for ideas.
Rowing exercises can build the muscles of the middle of the back.
Muscle imbalances can cause big problems, because subtle muscle imbalances are difficult to diagnose and take time and commitment to redress and rebalance. Most common imbalances result from aggressive bench pressing without concomitant work on the opposing muscles including the traps, lats, rhomboids and stabilizers, specifically the muscles comprising the rotator cuff. Most quality health clubs will array their cybex or universal machines in such a way that you exercise the protagonists and antagonists as you move sequentially down the exercise line. Do not skip machines. Stabilizer muscles like the rotator cuff muscles need to be specifically worked with very light weights, say three to five pounds, or elastic bands.