[jpluscplusm]

No SEO can guarantee results. They aren't the search engine, so how could they?

I'd be interested in the changes to your site, your data architecture and your content that they have suggested because, if that's not how they've worked, you've at best (or worst) engaged a bunch of black hat SEOs whose actions may cause you extreme pain down the line. At worst (or best, depending) you've engaged people who have taken your money for no action, but at least there's no reputational damage.

If you /have/ had proper engagement from the firm, you will probably have derived value from their work in the areas I mentioned above, regardless of your SERPS position.