If you are talking ballet I assume you mean a grand jeté. First one would need to see what you are doing incorrectly. That should come from your teacher who is watching your form. That being said here is the answer (tips) I give to most who ask about this. Remember this is general and not being able to see what you are doing limits how much this advice can really help you.
A grand jeté is not the same as doing a split, because even though it may look like it's related, it's not as connected as you might think. A grand jeté begins with a grand battement to the front, but it is combined with an extremely vigorous jump from the back foot and leg which transfers weight to the leg in front. As the dancer leaves the ground in the jump, the back leg also performs a sort of grand battement to the rear, but the battement should not cut into the flow of the leap. If you are trying to square your hips in the latter part of this step in order to get a perfect split in the air, forget it. It's not necessary.
And don't forget that a grand jeté must travel through the air, not just go up and split. In order to get the back leg up, the body needs to be moving up and OVER and continue moving