Does the title and length of a book influence sales?.

Last week I published the first of two Regency box sets. I already have two Duke Boxsets up on Amazon and these sold amazingly well -especially in the first few weeks. I was hoping for a similar result with my latest box set. Unfortunately this has not been the case - The Regency Boxset has not sold anyway near as well.
It has the same wonderful cover -is filled with three titles as good as the others -but has mad no impact at all. My conclusion -and I would be interested to hear your view on this -is that the title is the problem. A Regency without a duke in the title is never as popular.
I must elevate my lords to dukes for the fifth and sixth boxes - I was intending to call them 'Lords & Ladies Boxset One & Two -but maybe not.
As there were only around a dozen dukes around in the Regency it's amazing how many there are in fiction! In order to be a duke the family had to have been related to a King/Queen at some point in history.
Another strange thing that has happened is this. My sales in the US have fallen recently - used to be 60% US -and is now 30% - but my latest release, A Most Unusual Governess, has taken off in the US and not in the UK. The only difference I can see is that this book is a novella and nine of the others are full length books. My other 'best seller'  in the US was also a shorter book.  I'm hoping this is the case as my next few books will all be novella length.
Next week Anna Jacobs will be guesting here - can't wait!