There may be an instance where there may not be much user interaction on the site (not much activities from your visitors) and that could also be another contributing factor that would slow the crawl rate on your site.
The spiders take many things into consideration, such as your site authority and importance, and if your site has plentiful of interaction, in terms of people commenting or repeatedly browsing the pages (low bounce rate), these are key indicating factors that your site has something of interests and the spiders would be more active on your site if they do detect activity.
One way to increase the crawl rate is to build quality links from quality site, and with a bump on authority, the spiders would be more interested on your site. This should be gradually developed over time, and never neglected.