People have to be in the right place mentally to hear what you are trying to tell them. Otherwise cult programming rasserts itselft and they just repeat the same old platitudes and are reassured again.
Try to connect to the authentic person rather than talk about doctrine. Ask if they were not in the organisation what might they have done, what career may they have pursued etc
Talk to them about other high control groups, the techniques they use etc.
Look at threads here where people have successfully engaged with witnesses and the tools they have used.
If they mention doctrine to you ask them to explain it. Don't tell them stuff ask them to explain stuff to you. Don't let them gloss over the facts or uncomfortable truths ask them to provide evidence to you.