Here are the possible reasons of why custom form is not showing:
1. Log-in to your Admin area and go to SETTINGS-->CUSTOMIZE-->CUSTOM FORM. Make sure that the fields, you created exists and assigned to service you selected from the client booking interface.
2. Make sure that you are not logged into the Administrator area within the same browser window where you are testing "Customer Booking" area. This will never happen to your clients. If you want to test both in parallel then open both in separate browser. (For example, Open Admin in Firefox and Customer booking screen on Google Chrome).
3. "Custom form" is a paid feature available in PRO+ accounts. During trial it is available. Make sure that you do not run out of trial. After trial your account will downgrade to Free version that allows unlimited bookings, staff and services but not "Custom Form".