Problems with "switch to stage if mail received" action

This action works together with the "send message by email" action. It is known that the subject of the letter should contain the process ID. We specify it, but when the letter comes to the mail, when answering it, the action with switching does not work and the letter remains where it was. I would like to clarify that in several days of testing, it was found that the action works periodically, but only once, and if you try to repeat it with other deals, the action no longer works. Even with the deal she worked on.