Below are some of the most common reasons why a view is not displaying the cards you look for.
Wrong Project / Team selection
The cards being displayed on a view belong to the projects and teams that are selected in the view settings. When working with a few projects the project abbreviations for the selected projects are displayed at the top. If you have many projects selected, there will be a number representing the count of them.
Check to make sure that you have the correct projects and teams selected. If you are looking for items that are not assigned to a team yet, be sure to include the No Team into your selection.
The projects-teams selector is a part of view settings. Initially users see views with a selection set by the view's owner. Later they can change the settings for themselves only by clicking on the selector. When you apply personal custom selection for a view, the selector will be highlighted yellow, like here:
There is a handy 'revert' option to quickly sync the view's projects and teams from your personal settings back to its default settings. For more information see the article "Projects and Teams Selector".
Incorrect Filters Applied
Multiple filters can be applied on the views. The first filter to check is the Quick filter. You can see it above the view and it filters cards that are on the view. The Quick filter is your personal filter. Therefore if your colleagues open this view having some other Quick filter applied, you might see different results from them.
Inner filters can also be applied in the Set up view -> Setup tab. These filters may hide cards, rows and columns from your view.
If a filter is applied for items that are in a current iteration, make sure that there is a current iteration in the project. The same for a release and a team iteration. Current iteration and release should have start date earlier than today and end date later than today.
Incorrect Card Selection
Verify you have the correct types of cards selected to display in the view settings:
Views are split by pages when there are too many rows or columns to be displayed. This means that cards might be on the view, just not on the page you are looking at. You can browse pages using the arrows.