Depending on the submission service and type of submission method... usually, they say that most of them are not worth the money.

If you are still getting traffic, and haven't noticed too great a fall  in your SERPs, then it might be something else, like freshness of content that could be hurting...

My first site is http://www.1031store.com and it is a very seasonal type of site.  Although it is about real estate investing, many tax sales (which is the type of transaction the site specializes it) occur towards the end of the year.

Since I've been tracking the search terms/keywords that I'm Seo'd for, I've noticed that they increase/decrease according to the time of year.

Another thought is the "sandbox" effect that google has for links and sites.  Theoretically, Link relevance and importance increases over time.  Additionally, many sites that go through a link submission service trigger a sandbox penalty in addition to the normal sandbox timeline.

What this means is that you may be seeing the fruits of your submission effort now, a year later from your initial SEO efforts...

Just thoughts, you'd need to be the man at the different search engines to really be behind the scenes.