Custom fields not shown on Step 2 of 'Merge Duplicate Accounts'

When merging duplicate Accounts and one of the fields is not listed on Step 2 (Name of the screen - Select the values to retain) page, then the next step is to edit the layout for the Account and make sure that the field in question is visible on the layout. This can be achieved by going to:

Classic: Go to Setup | Customize | Accounts | Page Layouts
Lightning: Click the Gear icon then go to Setup | Object Manager | Account | Page Layouts

If the records being merged use more than one layout, then Users will need to add the field on at least one of the layouts.

This is because the fields available on the Merge Duplicate Accounts page (Step 2) is a combination between the fields available on the layout or layouts of the records being merged.

Here is an example on how this works.

Scenario 1:

 

Scenario 2:

When merging Record 1 - using Page layout A and Record 2 - using Page layout B
Field A is available on Page layout A but not on Page layout B
Result: The fields available will be a combination of the fields available on both Page layout A and Page layout B so Field A will still be visible on the Merge Account layout

Note: In Lightning, when clicked on all Show All Fields, displays all the fields which User profile has access to, through Field Level Security.