Practise while going over a fence then make it smaller and smaller till your just going over a pole and then if he’s good at the remove it
try different ways to encourage changes. maybe he is just unbalanced
He could just not be very balanced. Be patient and work on them every time you ride and try to be consistent!
Start by practicing lots of simple changes correctly across the diagonal as you would a flying change. Make sure your body is positioned correctly when you ask for the correct lead of canter across the diagonal. Make sure you and the horse are balanced. This will help get you both in position for a proper lead change :)