We have 3 web dashlet's on our dashboard. Sometimes the dashlet load properly and sometimes it does not show up without refreshing the page. Is there a refresh time on that page?
If you have an RSS feed dashlet on that page, disable it or make it the very last dashlet that loads. For example, if you have a 3 column layout, move it to the end of the 3rd column. I have no idea why but when certain external RSS feeds are consumed, it messes up the page loading sometimes eliminating all dashlets that come after the RSS dashlet.