Facebook does not allow most mobile devices to access the content within your Page tabs, where giveaways are hosted. To ensure that all of your users are able to access your giveaway, you can use the mobile friendly link provided in the 'Add to Facebook' section of the Publish tab for your giveaway and is also visible when viewing the content of your Facebook page tab when logged in as the admin of that page.
This link will direct users on desktop and laptop computers to your Facebook Page tab. For users that click on the link using a mobile device, they will be directed to your hosted page, where they can enter the promotion.
**Please note that if you have enabled the Facebook Like Gate those users accessing the promotion on a mobile device will not be shown the 'Like' gate prior to accessing the entry form.**