A user sends emails to a list it is a member of in List manager. It can happen that this user sends an email from any other email address that could be 'A Shared Account' or 'alias mailbox', meaning it is other than the one which is listed as a member of the particular list. Then this email is not processed by the List Manager to be sent forward to the members of that particular list.
This issue happens because the email (mailbox) through which the email is being sent is not listed in the particular list. The list manager does not process emails from email addresses that are not added to the target list. This issue can be fixed by adding the user's email address variation as a member of the particular list.
Refer to the following article to add user email to the list:
Note: Emails from unmoderated members of a list are processed and sent forward by LM without waiting for approval. So remember to set the user email variation as part of unmoderated senders also after adding to the list if the user was originally an unmoderated one. This help guide link mentions member moderation.
After adding the email address variation as a member of the desired list, try resending an email from the shared account or alias email address. The incoming email should be processed to be sent out by LM and reaches the recipients in the list successfully.