Actual FG higher than BeerSmith estimated FG

I have a bock beer that has been in the fermenter for 12 weeks now in cold storage.  The FG when we tested was 1.026 and Beer Smith estimated that the FG should have been 1.015.  The yeast used was Bavarian lager Wyeast 2206.  I plan on letting the beer sit in the fermenter for another week, then test the FG again to see if it drops.  But just asking the question now, if the FG is the same, should I add more yeast to get to the estimated FG or should I bottle at the FG that I record next week (assuming that it does not go down at all).  Also, if I should add more yeast, what type of yeast should I add, how much should I add, and what should the temperature be to for fermentation?

What was the OG and what temp did you mash at?