If subscription order is initiated by customer on the front-end using manual payment or Spree::BogusGateway, and admin approves payment from the admin dashboard then the Order state is changed to "Complete" but subscription is not updated.
If Admin panel is changing order to "Complete" through ActiveRecord callbacks then State Machine is not invoked, so below is never triggered:
self.state_machine.after_transition to: :complete, do: :enable_subscriptions, if: :any_disabled_subscription?
See similar issue: https://stackoverflow.com/questions/22622254/spreeshipment-state-machine