This video will describe how to adjust the needle depth of your Redline Embroidery Machine. If you don't have a needle depth gauge you can use a 6mm Allen range to measure the distance from the tip of the needle and peg of the rotary hook.
to buy the needle depth gauge go to: http://redlineemb.com/store/index.php?route=product/product&path=70_59&product_id=614