The Follow Up Ninja is a hybrid automation system, designed to automate campaigns but be recieved as human interaction.
There will be times when a lead responds to an automated text, which triggers the Follow Up Ninja to end the automation and let you take over. (See "Triggers" in your Settings to see what triggers the end of an automation)
Some instances also occur when a requested call to action (e.g. "reply YESPLEASE if interested) is not responded exactly as requested. This would require you to manually step in and take over the conversation.
Once you are engaging with the lead manually, you can take it from there.