Do you have sliding plates on your horse's back feet? A horse cannot slide comfortably or efficiently without them.
Do you have good stopping ground for him to slide on? Hard packed or rough ground or deep ground make it difficult or impossible for a horse to slide nicely. Good 'stopping ground' is absolutely necessary.
Does your horse give you its head while loping on a light contact? A horse must be collected, must tuck its chin and elevate its shoulders and lope with its hind end well up under it in order to slide when it stops.
Are you 'pulling' on the reins to 'stop' your horse? The horse must be stopping because you have trained him and have 'asked' him to stop. If you pull on him to 'make' him stop, you get a stiff legged bad stop with a lot of resistance. You MUST teach a horse to stop, then 'cue' (ask ) him to stop and then 'let' him stop. Anything else results in a bad stop.