Are your groups with broadheads hitting the same point of impact as your fieldtips, but just a bigger group? Or are they still grouping well, but not where your fieldpoints are?
I'm guessing, by your explanation that they're just all over the place. Most of the time this is becuase your broadheads aren't lined up with your arrows. How fast is your bow shooting? The faster your bow, the more misaligned broadheads will mess things up.
What kind of arrows (make and shaft size/spline), fletchings, and broadheads(weight also) are you using? Have you spun them to make sure the broadheads are in proper alignment to the shaft.

There are lots of variables that could come into play. The more information you give us, the easier we can help.

peashooter