bigcountry's right on. A bright but cloudy day is best. I carry a tape measure with my Chrony and usually set it 10 ft. from the muzzle. The velocity is measured by the time it takes the bullet to pass over the photocells, and if it's too dark, they won't see it well. If it is used in bright sunlight, the same thing can happen Does yours come with the shades for over the photocells? I usually find it necessary to use them on most of the days I shoot.
As far as the windage change, I've never had that happen....
Don't be alarmed at lower velocities than companies publish. A lot of them are taken from barrels longer than the general hunting rifle comes equipped with, and some just plain seem imaginary.