I agree with the time of year comment due to the fact that a company I've previously worked for (Restaurant industry) said that weeks in November are its quietist.
People are looking to buy gifts now, few people buy cars as gifts.
Also maybe being RWD scares people off of BMs at this time of year?
Lastly, yet something I thought at first, is the colour of your car. That maybe hindering the sale. I'm after black again next and maybe most other people are?
I remember a lad years back worked at Mercedes in the parts dept and was spec'in his Merc. He requested red leather and the dealer he worked for wasn't happy as they said it would knock a couple of grand off its future value when they had to sell it.
People are fussy. The majority of BMs and Mercs are silver or black. However boring that maybe.