Usually I can keep my heels down quite well but I struggle to keep my lower leg forward and heels down over jumps. Any advice on how to help fix this?

Try using no stirrups and keeping your leg still and in the position, also try bareback jumping if your ready for it because it will help u use your leg muscles for grip

drop your irons

Do no stirrup canter and trot work to help your lower leg stay still. It helps me when my trainer yells at me right before the jump to keep my heel down and my lower leg forward

Bareback at the canter works wonders.

Jump without stirrups or do it bareback ! Improved my jumping position loads ! 🙂