All of the problems upset customer relations and cost time, money and good will. But the angle of the mitered joints seems to be miscut. The openings developed during the early summer months. Fluctuations in relative humidity drives dimensional instability of wood. Wood shrinks and swells most in the direction tangent to the growth rings and about half as much perpendicular to them.
Dimensional change is insignificant in the direction along the grain.
A dry piece of wood casing, tightly mitered and installed during the winter months can look much different during the summer months. Picture what happens to the casings as the humidity in the home swings. But it is the way they grow that causes the problem. Because wood swells by different amounts in each direction, mitered connections remain tight at the bottom, but separate at the top as the casing swells. Lead the customer on a careful walk-through after the job is completed.