There can be a wide range of causes for such symptoms. For example I just spoke with a client who released a message from quarantine, but when the SaaS Cloud attempted to deliver the message it was denied by the exchange server for being over size, but I have also seen Exchange level content filtering kick a message back, and even issues such as Backpressure.
My recommendation in this case is to contact the support organization that works with your account and investigate further to find the root cause.
However, I can say that regardless of the error that is seen when a message is released, once it is released from quarantine it is treated like all other email and isn't retained on the system and would need to be resent. Finding if an error is being encountered after the release will be the first and most important step.
I've added a feature enhancement request for being able to release without the delete action. In the meantime, be sure to open a support ticket to find out why your releases aren't getting through.
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