I'm sorry to hear that your contacts are not all being added to Constant Contact from MindBody. This integration is built and managed by MindBody, so they would be the best team to help you resolve this error. In the meantime if you can export your contacts from MBO then you can upload them into new lists to make sure you are sending to the contacts that you want.
Here are instructions for importing a file of contacts.
Please reach out if you have any further questions!
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