There several things that could prevent a subscription order from being processed when it becomes due. Below is the checklist that Modular Merchant's Tech Support team starts with when investigating unprocessed subscriptions. This checklist has proven to be very affective in diagnosing subscription problems, so we've posted it here for our clients to use too. Let's review each item that can prevent subscriptions from being processed:

Select the time(s) for your subscriptions to be processed in the module's options.

In the sample image above, the store will attempt to process any subscriptions that are now due every day in the 5:00am hour. Note, the store won't process the subscriptions at exactly 5:00am — a random time within that hour will be selected by the system. This prevents potentially overloading the server by trying to process every store's subscriptions at the top of each hour.

Why select more than one hour in the day to process subscriptions?
Selecting more than one hour is optional. Some clients schedule more than one hour in the day so that they can correct any declined orders that occur in the first hour so that those orders can be successfully processed when the system reruns again at the next selected hour.

1. Log in to the Plesk control panel using the instructions located within your store's Administration Area at: [Hosting > Connection Information].
2. In the Plesk control panel, click on the Domains link.
3. On the Domains page, click the link for your domain name.
4. Click on the Scheduled Tasks icon in the "Additional Tools" section of the page.
5. Click the link for your Plesk username in the System User section of the page.
6. This will load the Crontab Tasks page, which lists all of the cron tasks running on your website.

If no cron tasks are running on your website, the screen will look something like this:

A crontab with no scheduled tasks.

Otherwise, the screen will list the cron tasks you have running, which will look something like this:
 

A crontab containing a scheduled task.

If your website doesn't have a cron task ending in "cron_manager.php", contact Modular Merchant support, and they will install all of the store's cron tasks for you ASAP.

If a subscription is assigned to an incompatible payment gateway, then a warning message will be displayed on the Search Subscriptions page in your store's Administration Area at: [Orders > Search Subscription Queue].

Any subscription set to use an incompatible payment gateway will be declined when it becomes due, and it will remain on your list of subscriptions, unprocessed.
 

When this happens, it's usually easy to track down. Declined subscriptions will be included in the Declined Orders report, located in your store's administration area at: [Reports > Declined Orders]. Mousing over the icon in the Decline Reason column will display a popup listing the specifics about the declined transaction, including the payment gateway's reason for the declining it.

Most of the orders declined in your shopping cart will be listed in the Declined Orders report.

If a subscription is declined by the payment gateway, then it will remain in your Subscription Queue, and the system will attempt to process it again the next time your subscriptions are billed. When the subscription is eventually successfully processed, it will pick up right where it left off. No orders will be dropped, and there will be no need to manually adjust the subscription's bill date.

If the system attempts to process a subscription containing products that no longer exist in your store database, it can raise a security flag which will prevent this or other due subscriptions from being processed until the suspicious subscription is corrected.

Custom scripts that declare functions can slow your store to processing just one or two subscriptions per Processing Time. This is because, when the first due subscription is processed, the custom script is executed. But, when the store attempts to process the second due subscription, the custom script is executed again, which attempts to redeclare the function the custom script contains. Attempting to declare a function with the same name more than once will cause a fatal error in PHP, which causes PHP to stop processing the subscriptions.