This is a reply I received last season regarding a complaint about tranny and bias.
Thank you for contacting The Football Association regarding the incident involving David Dunn and Cristiano Ronaldo during The FA Premier League match Manchester United v Blackburn Rovers on 21 February.
The referee Howard Webb has confirmed that he saw this incident and that he dealt with it at the time. Therefore no further action will be taken against either player.
FIFA rules prevent the "re-refereeing" of matches. FIFA are the world governing body of football and The FA is just one of 208 member associations, all bound to the same set of rules. This particular rule prevents The FA from taking disciplinary action on incidents which are seen and dealt with at the time by match officials (this includes the referee taking no action). Therefore if the match officials confirm to The FA they have seen an incident, The FA cannot act retrospectively.
IMO, the FA are trying to find a way around their own rules as they will have received Clattenburg's report by now. If he'd said that he'd missed the incident I'm certain that there'd have been diciplinary proceedings announced already.