You may have hit his shoulder blade.If he was laying down for 20+ minutes and all you found was moderate blood.I would check to see if you can get a tracking dog.Also like someone already mentioned,they blend in very well in a cornfield.He may be 5-10 yards off the blood trail.
Almost anyone who has done alot of hunting have wounded deer and not been able to find them.I know how awful you feel.I've been hunting for 40+ years and have wounded 2.I saw one of the buck's the following year and that made me feel great.Please keep us up to date,Good Luck!