This can happen if the door is installed above the cat’s belly height, it is too high. You can add a small step to allow the cat to enter at a straight horizontal angle.

Also, check that the key is staying in front under the cat’s chin.

If the cat is trying to open the door with its paw, the key may not be close enough to the door. Try training the cat to push through the door with its head by tempting it with food.