Broadheads are like a problem amplifier. A minor glitch in shooting form that will still yield good groups with field tips will make itself glaringly apparent when you screw on a broadhead. The reason is that the fletchings can easily correct an arrows flight with a field tip on the other end buta broadhead is like another little set of mini-vanes fighting against the fletching for control of the arrow. If your broadheads were consistently hitting high/low, or left/right of your field tips then you would obviously have a bow tuning issue. If your groups are just opening up in general I'm betting it's a shooting form issue.

The next time you shoot concentrate on consistently placing your hand on the grip the exact same way each time. Do not torque or grab the bow. I shoot with an open handed grip and feel that it is the best way to insure that you do not torque/twist the bow or grab at it as the shot goes off. Remember that arrow is still on the string for a few milliseconds after you hit the release so I hold the pose so to speak until my arrow impacts the target.

Also not all fixed blade broadheads have the same flight characteristics. Some are far better and more forgiving than others. I would recomment trying some Wac'ems by Triton or Slick Tricks. I have fouind them to be two of the most forgiving fixed heads out there with both offering almost field tip accuracy.