My horse used to be really lazy. I halted him and did sharp harder taps behind my leg close to where my heels are until he trotted. This helps because it's more "annoying" than just one bigger one and in that location they associate it with not listening to your leg. Repeat a few times and your horse should get more responsive 😋
I find a dressage whip very useful to engage the hind quarters
Don't always apply pressure because then they become destined
maybe a small pair of spurs or carrying a crop!