The subscriptions are managed by Apple or Google and this can happen for a number of reasons. When it happens, Apple or Google will usually follow up with an email to your App Store/Play Store account email with a reason and instructions on how to fix it.
In nearly all cases, though, it's usually just an expired payment method or some other payment issue and the payment method just needs to be updated. See here for instructions:
Contact Us
If that doesn't resolve the issue, or if you have any other questions, please contact us via the Contact Developer option in the app's menu or at support@flightcrewview.com