Email shopify@directory.com.au and include your store's .myshopify.com address.
Every published form has a page at https://your-store.com/apps/forms/f/<handle> (the Embed tab shows the exact link). Link to it from a menu, or add the "Custom form" block to any page from the theme editor and type the form's handle into the block's settings.
Check the field rules (required, minimum length, number ranges) and whether "customers must be signed in" is on for that form. The form shows the exact reason beside the field.
Prefill only applies when the visitor is signed in to a customer account on your storefront. Each field's "Prefill from" setting picks what fills it.
Use Settings > "Send a test email". Check the spam folder, the recipients on the form's Notifications tab, and the store default under Settings. You can also send from your own SMTP server so the emails come from your domain.
Open the entry: the Shopify panel shows why. Usually the permission has not been granted yet (Integrations tab > Grant access), the email was missing, or company registration is on but the store does not have B2B (Shopify Plus). Fix it and click Retry on the entry.
The store has used this month's submission allowance (20 on Free, 100 on Starter). Held submissions are stored safely, files included, but are not shown, emailed or synced until you upgrade; then every held submission is released as if it had just arrived. The counter resets on the first of the month.
The store has reached the fair-use storage ceiling. The Plan page shows what is used. Delete old entries to free their files. Submissions keep working either way.