The question is, what do you currently rank for. If you're ranking for solid purchasing keywords, and you've got a solid/stable position, you might want to look around at your competitors (e.g. those above and below you), and see what they're offering that you're not. However, if you're ranking for some obscure keywords which really have no commercial intent, that's probably why you're not getting any requests.
In web design, or custom services in general, it's not about pushing funneling conversions directly to a sale (e.g. a buy now button on the homepage), but more about delivering the information the potential client needs to see (services + pricing ideas, previous works + references etc) in an efficient manner and easy flow.
You clearly have a decent background when it comes to web design, but compare the quality of your site, to these;
No offense intended, but your website is pretty 2007. It may be fine for shifting the unsavvy offline business (potentially, all though dealing with them will probably cause you a lot of headache, which is why I also recommend you implement some kind of step-by-step walk-through/full request for proposal form).
I'd recommend getting inspiration from
Forrst, it's like a repository for designers who want to inspire others.
Bottom line is though, unless you've got traffic from a solid keyword source, you're not going to get many inquiries. I'm not sure what the average rfp:visitor rate is from search, but what I do know from a first hand experience is - web design is a very competitive business, especially in services.
