One thing that can cause this problem is if a user marks any message received from Groups.io as spam.
There now exists a requirement on the part of many e-mail service providers, utilizing a feedback loop to Groups.io, that if a message is marked as spam that Groups.io must cease sending messages to the address from which the spam marking originated. This is wreaking havoc particularly since the marking as spam is accidental.
If you are receiving individual messages as your subscription and don't want to get any more messages for a specific topic, do not mark it as spam, use the "Mute this Topic" link at the bottom of the message and no more messages from that specific topic will be sent to you.
It is also possible to arrange, via the Groups.io web interface, to have the first message from every topic sent to you but, if you do not choose to follow that topic using the link that's provided at the end of the message, no more messages from that topic will be sent (unless you were to choose to follow it later, either via activating the follow link in the first message that you didn't delete from your inbox or going in to the web interface to do so).
The real art of conversation is not only to say the right thing in the right place but to leave unsaid the wrong thing at the tempting moment.
~ Dorothy Nevill