You want the honest truth? If the peep can move then your string doesn't have near enough twists in it. Since I started getting aftermarket strings I've come to realize most factory strings should have about 5 times as many twists as they do. Of course that would make them pretty short in the end, which just proves they aren't made right in the first place.

My strings come with a twist about every 1 1/2". Put a peep in and this gets divided in half so there is only about 3/4" gap on each side so no tying is necessary. And I have a Meta peep, Fletcher and Superball. All get the same treatment---nothing.