A form shows submissions in Account Engagement but no new prospects are appearing. How would you troubleshoot?

First I would confirm the premise — are they genuinely not being created, or are they being matched to existing prospects? If submitters are using email addresses already in the database, the submission updates the existing record rather than creating one, which looks identical on a prospect count.

Then: is the email field mapped and required on the form? Without an email address there is nothing to key the prospect on. Is kiosk mode enabled, changing how submissions are attributed? For a form handler, has the field mapping drifted because the developers renamed an input?

I would submit the form myself with a fresh address and follow it through — check the form's submission report, then search for the prospect, then check the Recycle Bin in case a rule is deleting them. Prospect Audits on a test record shows exactly what did and did not happen.

I would also check for a completion action or automation rule doing something unexpected, such as adding to a list that another process cleans out.