Welcome to the Q&A Forum

Hi, I appreciate this must be very frustrating for you!

There are any number of reasons why you could be struggling to generate organic traffic.  My first port of call would be to check your Google Webmaster Tools to check if you've been hit by a manual penalty before you do anything else.

When you say "Content is added regularly", what are you doing to promote that content?  It's unlikely it's going to help you rank on it's own if people are unaware of it.  Also, does your content have that "wow" factor which people would be inclined to share?  Find yourself a unique standpoint then promote your content through channels where your customers are online.  Try not to churn out content for the sake of having content, you're (arguably) more likely to have success by really concentrating on content (so, it doesn't have to just be text in a blog) which can't be found anywhere else and provides value which your competitors can't easily replicate.

How close does the dip in traffic correlate to the new site design going live?  Was it just a re-skin or did URL paths and/or content on the site actually change?  In addition to checking your Google Analytics (or equivalent) Id check Webmaster Tools for the Search Analytics report to see if any loss in traffic from specific keywords can be tallied back to the site launch.  This could be more of a technical issue than a content one if so.

It could still be external factors of course, have you been tracking any of your competitors, have they been more active of late?

Losing traffic is very different to struggling to get traffic in the first place, so thanks for clarifying!