i got some problem in my batting technique in cricket need advise from experts?

[ash]

i love playing i have been playing it from more than 10 years i am by the way 18 i have good timing and can find gap easily.the only problem i have is i cant hit the ball high in the air i am not talking about sixes the only way i have sixes is by playing straight and still it used to go flat.. i want to learn how to hit the ball high in the air and i cant hit length ball for six i dont know why i can hit only full length deliveries please some body help me with my technique