Sounds like a Hostage rest- same I am shooting. I have the pro version which is aluminum and has incremental marks to make fine adjustments. There are much fancier rests out there but I am only a hunter and it gives plenty of adjustment for me.

If it is that rest, you have another variable with the brushes that hold the arrow. As you shoot, the brushes tend to wear like a toothbrush. Consider replacing them if your groups open up, or if you are in the field, just rotating them around will make a difference also. Usually it is the bottom one that gets the most wear.

Hope any of that helps.