Assuming there is no mechanical problem with the horn, my guess is that your embouchure is too tight and/or you are taking in too little mouthpiece.

Try playing just the mouthpiece and compare the pitch when the registers are "in tune" to your normal embouchure.  

When you find that pitch practice holding it with just the mouthpiece for a few minutes a day. Then go to some long tones concentrating on the embouchure.

It shouldn't take that long to correct if you go slow and concentrate on just the one correction.

You should not have to change embouchure through the registers to play in tune.