Cornus Alba Siberica stems will begin to go brown as the stems mature. It's the new stems which are produced this season that will be red. Yes, seasons and soil type can affect the colour but many people will cut the old stems down in Spring to produce new ones. They do like damp, moist soil though.

As Jimmy says it is the natural colour of the more mature wood. The bright stems are seen best on the current years growth. so next winter the lower part of the stems will be browner with this year's wood red. I prune a 1/3 of the stems down to the base every year [evenly spaced] so I get a succession of red stems.

Agree, only the new twigs start out red, then turn brown. You can give it a trim, then the newer growth will be red.

I normally cut mine down to within about a foot or eighteen inches of the ground before the leaves open, because I only planted it for the red stems.