If the sample period is only a few weeks, it's not really enough to draw any conclusions about the shift or fall in sales, so it's only possible to theorise. 

New listings do get a boost in best match results, but if you're still ranking highly, then there's several other possibilities that could have caused it - eg brand popularity and representation on the site. I don't know how many other sellers stock what you do, but if you're a newcomer with good prices, have stuff other's don't, etc, it's possible people bought specific things they've been keeping an eye out for once you put the listings up.

If your listing format / duration changed, the sense of urgency to buy can fade (i.e. if you had listings ending within shorter periods, people can be more inclined to purchase than when they see the seller has a store, and the listing is likely to be around for a while).