i see,but rasengan is very direct ninjutsu,practically an extension of a signature open-palmed Jyuken strike in his taijutsu. i dont think shape manipulation should be a problem with a master of the "gentle fist"as rasengan is very delicate and doesnt use hand signs.
it seems almost like a plot hole that he doesnt at least use a similar attack in the seriies but im just pondering a theory within a fantasy!
edit:and i understand it is not easy to use but i meant relative to other shinobi it would be easier for the Hyuugas.