Make sure they are attempting to reach the correct page. The URL address should be: https://vote.associationvoting.com/xxxxx/ where xxxxx is to be replaced by your election acronym, generally the abbreviation for your organization.
If the voter has never been to an associationvoting.com website (which includes your election site), you can ask them if there is a trailing / on the webpage URL address. If so, ask them to refresh or reload the page.
For an iPhone never having loaded an AV website, if it has the https:// at the beginning and a trailing / at the end of the URL address (https://vote.associationvoting.com/xxxxx/), it should work every time.
If Safari only doesn't load AV pages (you can reach other websites), you can remove data related to that site to see if it fixes the issue. Use these steps to remove cookies, cache, and other data stored by Safari for a specific site:
If Safari doesn't load pages from multiple sites, but does from other multiple sites - In OS X Yosemite, you can delete website data for the past hour, or past few days if websites or pages stopped loading recently.
If all else fails, you may need to check with your network administrator to see if you can get access to the site that you're trying to view.