Unable to process queue

Hello – I am using this plugin for the first time and emails added to the queue do not send when processed manually, and never send via chron either. They just stay in queue with no errors. If I send a test directly without waiting for chronjob to process, it goes through as expected. Unsure what to do about the emails stuck in queue. Thanks for your assistance.

https://wordpress.org/plugins/smtp-mailing-queue/