there are seals to the rasengan, its just that naruto doesnt use them and the sharingan cant copy it just by using it. as we saw, the sharingan predicts movements, its not some sort of majic eye that can breakdown down and interpret a jutsu immediately. the reason why the shraingan "copies" jutsus is because the user copies the seals required. there are also multiple seals to the kage bunshin as we saw in the haku vs. naruto/sasuke fight, but naruto usally only uses the last seal (which is not one of the standard seals learned its probally a secret one like that strange finger twisting seal haku used for the ice mirrors) since sasukes never seen the full kage bunshin seal with the sharingan hes been unable to copy it. my guess is that seals heal with molding a correct amount of chakra and help it mold in a certain way to perform a jutsu. since the sharingan cant see the amount chakra needed, or how to mold it when full casting seals are used, it is impossible to copy a narutos moves when he doesnt use seals.