Facebook only shows some of your friends' posts in the News Feed - it is not known how it selects which posts will appear in the News Feed. In order to see all of your friends' posts, you have to create a custom list and put all of your friends in it. Then instead of going to the News Feed, you can go to your custom list and see all of your friends' posts.
To create a custom list:
1) Click on the FRIENDS section on the left sidebar.
2) Click on the "+ Create List" button at the top of the screen.
3) In the box that appears, enter a list name and then click the "Create" button. It will show the list you created with "No posts to show".
4) Click on the "Add Friends to List" button.
5) Click on all of your friends so that each person has a checkmark on their photo, and click the "Finish" button.
6) Reload the list by clicking on it under the FRIENDS section on the left sidebar, and it will show you all of the posts from your friends.
The only issue with using a custom list is that whenever you add a new friend, you have to remember to add that person to the list. You can add a new friend to the list by clicking on the "Manage List" button on the upper right of the list page and selecting "Edit List", and then change "On This List" to "Friends", and then you can select your new friend.