If you're struggling to find your subcomponent, find the Custom Object first (hint: it may be in the All items tab) and then expand the components to find the subcomponent. For example:
​

If you are looking for a Custom Field, Validation Rule, Business Process, WebLink (Custom Buttons) or Record Type in the comparison results, note that they are classified as subcomponents of the parent Custom Object. (In the Salesforce metadata API terminology, Standard Objects are classed as Custom Objects.)

To retrieve these subcomponents, you will need to select them (and the Custom Object that they belong to) in the metadata filter:

As they are classified as subcomponents of their parent Custom Object, your item may only be present in the results as a nested component rather than its own top-level item. This is particularly likely if the subcomponent is No difference, as we make the comparison results more concise by only showing you the most significant items in the main view. If the subcomponent item is New, Deleted or Changed, it may be promoted to being a top-level item.