Hi Karen: Not sure I understand....but let me tell you what I think you said....
Your shallow pitched rear shed dormer has a finished ceiling that attaches to the 2X6 rafters. The rafter space is insulated but without a vapor barrier. You have installed a ridge vent and soffit vents. Yes?
If so, you have two potential probable problem sources that will cause the premature deterioration of your dormer roof. First, moisture doesn't shed very fast off very shallow pitches...so a 20 year roof on a 2 pitch roof may be in fact a 10 year roof if you're in a wet climate. This is why roll roofing or hot mopped tar is normally used on very shallow pitch roofs. Second, the lack of constant airflow under the roof deck is probably due to old insulation between the 2X6 rafters blocking the air. Unfortunately, on finished attics (like Cape Code style houses), the only way to remedy this is to tear off the interior ceiling, remove the old insulation and then install either foam 'chutes' to channel air from the soffits to the ridge vent, or do like I do and construct a chute using 1" X 3/4" pine rippings on each side of the rafter up against the roof deck and put rigid insulation up against that before putting up batt or blown in insulation...then reinstall and finish the ceiling. A lot of work...but if the attic is going to be a heated living area, particularly if your in a cold climate, I can't think of any other good way.
Hope this helps and isn't too discouraging (smile)