It seems some players just have this 'air' about them that makes desperate managers think "I'll take a chance on him!" only to discover the reality is not quite up to the dream. Some managers have that about them, too!
Rhodes is a bit of a Norwich City; almost too good for the Championship but not quite up to the Premier League!
Ross (despite one excellent goal-scoring season for us) has also never really hit the heights for a player that has commanded transfer fees in excess of £10m a time (not to say that I wouldn't have him back at the right price, mind)
Rudy Gestede is another one. Always rated his potential at Cardiff and often wondered why he wasn't given an extended run and he proved that with Blackburn Rovers for a while. But then he got his 'big money' move and (in reality) what since? Not a huge amount for the fees paid, yet Middlesbrough have decided to take a punt on a player who didn't really cut it at Villa in either the Premier League or in the Championship
Dwight Gale another (as someone mentioned above) and I am sure there are more...