Fax credits are tied to the device that you bought them on.  Most often when customers are missing credits, it's because they bought a new phone.  In this case, your new device will not have your account information, and your credits will not automatically transfer.  However, there are two ways to get your credits to transfer.

Do it yourself:

When you get a new device Apple recommends that you transfer content (including your JotNot Fax account information) by doing a backup and restore.  This should transfer all your apps and all your data to your new device.  Here are Apple's instructions on how to do that:

https://support.apple.com/en-us/HT201269

Contact support:

If you prefer, you can contact our support team to let them know that you would like to transfer your account information to a new device.  They'll need to know your account information from your old and new devices.  The easiest way to send that is to send a support request from JotNot Fax on your old device, and a second support request from JotNot Fax on your new device.  The support requests generated from within the app automatically include the account information we need to transfer your credits.  To send a support request, open JotNot Fax to the "Store" tab (second tab from the right on the bottom tab bar), then tap the "Missing Credits? Contact Support" button.

If for some reason you do not have access to your old phone anymore, please send a support request from within JotNot Fax on your new phone, and include a fax number that you have previously sent a fax to, as well as the approximate date that the fax was sent.  That will allow us to look up your old account.  If you don't have that information available, please send a copy of your iTunes receipt from Apple that shows the credits that you purchased but are missing.  Apple typically sends receipts to the email address that you use to log into the App Store or iTunes.