On lead conversion mapping if you find any fields missing it could be because of a few reasons:
1. Field types do not match in both modules
2. The field was first deleted in the linked module (Accounts/Contacts/Deals) after creating it through the Leads module. Later you recreated this field, this time directly in the linked module.
3. Custom field is mapped with system field.
*Rare but found this when mapping a custom field to Billing City other than Leads' system field City.
Here are fixes you can perform:
1. Ensure field types match
2. Delete the field permanently in the Leads module and also from the linked modules then create it again from Leads. Before you save changes ensure you duplicate it for linked modules by ticking 'Also create for ... Accounts/Contacts/Deals'. This will automatically map it on Lead conversion but check ;)
3. Map system field to system field or custom field to custom field.
Hope it helps! Feel free to share any other reasons you are aware of.
Milton - Solutions to Success