there's a lot of different reasons that can cause a riders leg to slip back, but the pictures quite small and bad quality so I'm not entirely sure what you're doing wrong as "putting your heels down" isn't always the case. I'd try a lot of no stirrup work and having someone lunge you over a jump with out reins with your arms spread out to your sides like an airplane (lack of a better word.. xD)
That'll keep you from leaning on your horses neck while jumping which will keep the weight in your leg, and most commonly it'll force you to grip with your calve. Or at least that was the reaction of my leg when I first used that method. I hope I was some form of help xD
