There are several reasons why a sausage might be under-smoked. These can include:
Too much stuffing: Sausages hung too densely prevent the free penetration of smoke between them.
Too much moisture: Sausages should be well dried before smoking. Insufficient drying can result in water from inside the filling permeating the casing.
Too short: The sausage should acquire color after 3-4 hours.
Too low temperature: Too low a temperature can extend the time required to acquire color.