How to solve the "400 Bad Request" Error?
If some of your subscribers created through MailMunch forms are not showing up in your Constant Contact list, then you can check the log messages for those subscribers to see if there are any "404 Bad Request" errors. If you do see such errors, you can do either of the following:
Re-subscribe them through your MailMunch form again.
Add them directly to your Constant Contact list by logging into your Constant Contact account.
Why is this happening?
This is a known issue with Constant Contact's API, and is randomly generated for a very few subscribers.
What if I have a huge mailing list?
If you have a huge mailing list and are having issues with identifying the missing subscribers, you can export your subscribers list as CSV from both Constant Contact and MailMunch and cross reference them in MS Excel using VLookUp to find the missing entries.