Please note you need an admin account to fix this.
1. The field-level security of the custom field isn’t set up.
If users don’t have access to a custom field, it won’t appear in the Field settings.
To make it visible: Go to Setup > Object Manager > Select Lead/Contact/Account > Fields & Relationships > Select the custom field not appearing > Make it visible in the Field-Level Security settings.
2. Changes to the custom field settings are affecting data loading.
When you make structural changes in the Salesforce Object after you set a fixed value, the fixed value may not work and custom fields may not appear. If this happens, go to the Scan to Salesforce tab, then the gear icon, open Admin Settings, click the Field settings button, and click Clear field settings to delete the cache.