You should not be expecting your conversion rate to rise unless you've made changes to the site. Your conversion rate should remain constant. Now, if your conversions are not rising then you have an issue. Typically your conversion rate drops slightly due to economies of scale as your traffic volume increases. Almost always the number of conversions increases as your traffic increases. (Conversion rate is measured as conversions / sessions)
A higher volume of traffic does not always automatically lead to a higher conversion rate. Yes, the statistics are there to justify this but surely the quality of the traffic that is coming to your site also counts: the better quality traffic in terms of targeting, the more likely they are to convert.
How closely are you looking at the traffic that is visiting your site? Do you know how well-qualified they are? Are you using any tools to measure the quality of this traffic?
I do actually work for a web analytics company and we hear this problem fairly regularly. As suggested, a way of increasing conversion rates is to have better qualified, more targeted traffic coming to your website. This should help to raise your conversion rates.
[edited by: Receptional at 3:36 pm (utc) on Sep. 15, 2006] [edit reason] No urls please. [/edit]