It could be a number of issues. Maybe you are trying to play from your fingers and not your pronator and supinator muscles. That pattern should all come from your elbow. Like turning a screwdriver or opening a door.
It could be that you are abducting. You can't use two muscles at the same time. Well, you can and that may be the problem. Try not to abduct and instead use the shoulder to place the hand where it needs to be rather than spreading out your fingers. A little movement from the shoulder or elbow can spell huge real estate gains at the hand.
You may lack an in/out or up/down motion which would get your playing in your arm and not your fingers.
You probably have a combination of all the above.
Ask a knowledgeable teacher to watch you play. They will probably spot the problem immediately.
If your teacher tells you that you need more practice or you need to build strength and endurance or special exercises, run fast and far from that teacher. Technique is about physics, not brute force.