Starting with most likely (IMO): Grain crush to course (do you do this yourself or does your LHBS do it?), not sparging long enough, not sparging enough liquid, bad fly sparging technique, sparge water not hot enough, Ph problems. Even if you fix the first two or three your efficency will probably shoot up.
Theres probably a few more that I haven't thought of. Tell us more about your sparge (batch/fly, time, temps, mash tun, etc).