Stand behind your truck about 8 feet and look at each rail. It’s just like checking a picture on the wall and you can easily tell if the rail is level or not.
This rail is not level:
How to Correct
View the video below starting at the 1:50 time mark. Check to make sure your bracket leg is level and bend it if necessary.
With your brackets level you will need to add spacing washers so that the outboard rail lip is about 3/16" above your truck bed rail cap. The following video at the 0:27 time mark will show how to do this.