There could be a number of things at play, but as has been mentioned, checking the history of your lead when you complete the form will tell you a lot about what's happening. If the history stops part-way through the flow, then is it possible that something outside the program is affecting the lead? For example, could there be a trigger campaign that changes the lead partition, which would stop the flow? If you've done all of the above from the other comments, then your best bet would be to call support.
Hi Clark,
The next step would be to log a case with support. That will allow us to log into your instance and take a close look at what's happening.
John.