Why is my trumpet's fourth line D so flat?

I have a JTR-1602 (also called a Jupiter Tribune) the fourth line D's are really flat, and it is impossible to change my embouchure to get it in tune, but if i tune to that note, all my other notes are sharp. I have lost the Jupiter 1.5c mouthpiece that came with it. Should i buy a new Jupiter 1.5c mouthpiece? do you think that that would fix the problem?