This isn't uncommon at all. You need to tune your rest for broadheads. Fixed blade broadheads magnify the imperfections when a rest is out of tune. Actually any bow with any rest that is out of tune will not group broadheads and fieldpoints together. Tuning your rest will also mean that your arrow is flying straighter and you will get maximum penetration. If your arrow is not flying true, you lose penetration because all the energy of the shaft is not deposited directly into the back of the broadhead. Tuning can at times be frustrating but be patient.

download this PDF file and follow the directions in the Broadhead tuning section, it'll work for you.
http://www.eastonarchery.com/downloads.asp

Also, what you do NOT want to do is to shoot mechanicals because the other heads won't group. I would say the majority of hunter's that shoot mechanicals do it to mask a poorly tuned rest.