Then there might be a problem with their payment system. If they are using recurring subscription like this sub pack will renew month by month then they should pay the payment every month to continue their sub pack. Please make sure that there is no problem with the payment or the paypal email they are using.
I have discuss with our developer but they said this is not a bug if it is then your all seller will not be able to use sub pack. So, i will suggest you to check again the full system and confirm the payment is correct. Please let me know after checking.