If tied in correctly the loop should not slide when tension is put on it, like when you are drawing the bow. However you should be able to thread it up and down the serving for adjustments. This is why a loop or tied in nock is a bit better than a clamped on brass one. They are easier to make small adjustments to.

Make sure when you tie one you use opposing knots on the top and bottom.

Another thing that helps with string loops is proper form and drawing properly. You should draw your bow straight back, not up at angle. This could over time move your nocking point, or if not tied in properly the first or second time you draw your bow.

Paul