WooCommerce Version - if you've just updated WooCommerce and the payment screen drops off, please contact us immediately.  We regularly test our software against upcoming WooCommerce releases.  Large version changes (such as 1.6 to 2.0, or even 2.1 to 2.2) frequently introduce changes that break compatibility with existing versions of WooCommerce plugins.  Small version changes (2.2.0 to 2.2.1 or 2.2.0 to 2.2.6) typically don't cause compatibility issues for our plugins.  
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We strongly recommend contacting support or checking our Announcements before updating your core Wordpress or WooCommerce installations.
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Theme Compatibility - Occasionally a theme will load an incompatible javascript library that kills javascript in certain cases.  This can create an issue with the display of the payment window as it is initially set to hidden to prevent a 'reload flash' that can occur when the plugin queries Vantiv for your payment details.
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Occasionally we will see mobile / responsive issues that you can explore in greater detail in the support thread dedicated to this issue. If you're comfortable making changes to the active theme and can check to see if the issue disappears when you return to a default Wordpress theme, that can sometimes help sort out where the compatibility issue resides.  We can occasionally help with theme compatibility issues, but this typically falls outside of the scope of our support of the plugin and becomes a custom support project that is billable on an hourly basis.
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Plugin Compatibility - Certain plugins can trigger a similar reaction to Theme compatibility issues by introducing outdated versions of javascript libraries.  
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Additionally, some custom 'order flow' plugins that break apart the normal checkout process into multiple pages may need to be customized to work correctly with the plugin.  This may manifest itself as a failure to display the payment screen, or a failure to display order details on the thank you page.  
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Some 404 / SEO / Redirector plugins can also trigger issues.  These frequently are plugins that try to intercept requests for pages that aren't 'cached' in the database to deliver a custom error page instead of a generic 404 page.  Often these are SEO type plugins that are trying to improve your SEO by eliminating 404 'page not found' messages.  Since the /order-pay/ page is dynamically generated for every checkout, none of the pages exist prior to the transaction and these sorts of plugins can intercept the error pages and send the customer to the wrong place.  The end result in extreme cases is an order that never gets updated for the 'processing status' even though Vantiv records the transaction successfully and charges the card.
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Browser Compatibility - At this time there are no known compatibility issues with the following modern browsers: Chrome, FireFox, Internet Explorer, Safari. We also support a wide range of mobile browsers.  Older our outdated desktop or mobile browsers may cause issues, and rarely some users may browse the internet with a browser that disables javascript or lacks other modern features.  We can only offer support on the last few most recent versions of the four browsers listed above.
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Server Compatibility - Your hosting account must support the SOAP Client.  Without this the payment gateway cannot run.  
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On occasion we have come across other issues, mostly relating to slow performance on shared hosting accounts.  If your server is running too slowly, it can lead to timeouts in the communication process with Vantiv.  You may also come across issues if your hosting provider sharply limits available RAM.  You may be able to fix these issues using a php.ini file or by tweaking your .htaccess file to increase the amount of memory allocated to PHP, lengthening the timeout window, or making other changes to your environmental variables.