There might be some reasons for the sales decrease:
* 1 series is quite old and the 2 series is around the corner;
* 3 series coupe and conv are waiting the replacement, 4 series is an entire new chapter for BMW;
* 5 series is waiting its improved (at least for steering feel
) LCI version;
* 7 series is already updated, but 6 series is very close in price range and sportier, somehow more attractive
I think that in the second part of the year BMW will climb again on the first place