Typically if there is an item you can't find on your To-do, it's because you have filtered it down and some items are hidden.

If you have unchecked certain to-do items from your filtering menu, that item type will be hidden from your list. To ensure that you are aware that some items are hidden, the header will show you what you're currently viewing. 

When all of your items are displayed, the header will be empty.

In order to find an item, make sure you are either looking at your To-do without any filters or make sure the right filters are ticked in order to find the item you're looking for.

Best practice tip: Use the sorting slider to make it even easier for you to find what you're looking for, by for example sorting by status or client.