It's hard to say but I suspect that you do not understand preparing artwork for video, video formats and frame sizes. Setting the ratio for every comp to 16 X 9 is not what you should be doing. None of your compositions should say Custom in the settings. Use the presets and your best option for now is to use HDTV 1080 24, 25, or 29.97 fps for all of your compositions. 29.97 fps will give you smoother motion and reduce juddering problems with line art so that's where I would start unless you live in a PAL country where the video standard is 25 fps.
If you are creating your artwork in Illustrator use 1080 X 1920 for your art board size. If you are creating the artwork in Photoshop use 16 X 9 frame size that is at least as big as your composition - 1920 X 1080 pixels but is big enough so that you do not ever have to scale your artwork more than 100% but small enough so that the artwork is close to 100% scale at some point in the composition.
You must spend a bunch of time studying video formats, standards and the theory of motion if you are going to be a successful animator. This is not something that any but the very lucky just stumble into with success.
You also have to take a fair amount of time and learn how the AE UI works and how to efficiently move layers around and build composites. You have to get a good start on the basics or you will be forever wasting time and asking questions that you would know the answer to if you spent a couple of days going through the basics. Start here: Basic AE
Hi Mari Poom,
Did you ever complete your animation? Please let us know what you did to solve your issue.