Yes, you have spent time with them on proper movement and spinal organization, but it doesn’t sound like they are understanding the concepts or what to do.
At first it can be frustrating when the athlete is not understanding a concept.
Continue to break it down to the smallest concepts and they will start to get it.
Demoing or having them watch a video can help especially if they are visual learners.
Describing what the movement should feel like can aid if they are kinesthetic learners.
Understanding the movement expectation and Motor Control are the first 2 places to hit.