The most common cause for this is a delay in sending email notifications. Try disabling notifications temporarily and perform a test submission to see if it goes through. If not, if you are processing any other feeds after submission, like sending to a CRM, disable those feeds temporarily and test again.
Finally, if you’re not able to quickly find the issue like that, enable Gravity Forms logging:
The log lines are time-stamped, and you can find the gap in the processing to see where the delay is coming from. If you need assistance, please open a support ticket: