A recent report about JaMarcus Russell may explain why he didn't play up to expectation. He has sleep apnea. For people who don't know, sleep apnea is a condition in which a sufferer can stop breathing in their sleep causing a sufferer to not get a good night's rest and in extreme cases can be fatal.
Russell is, or was, a pro athlete and having sleep apnea makes it impossible for someone to play at a high level. This may not be the main reason he was arguably the biggest NFL Draft bust. Russell could have had a better work ethic, gotten into shape (which could have improved his sleep apnea), and his then coach Lane Kiffin giving him no support. Had JaMarcus Russell stayed for his senior year at LSU, he could've been better, but we will never know.
JaMarcus Russell is now living in his hometown of Mobile, Alabama and has built a 50 yard practice field on his property in order to get back into the NFL. Russell is only 26 years old, so technically still has time left, but given Russell's tenure with Oakland and his "purple drank" controversy will anyone want him?
You have to give it to JaMarcus Russell, he is now trying to better himself. Had he done that in Oakland he might still be in the NFL, probably not a star, but in the NFL nonetheless.