I'm not saying this is you, but consider that you might need to stick to superior weapons (i.e. rifles) until you improve. This is my seventh year hunting. So far, it's been rifles only (except turkey, but I've yet to seal the deal).
Next year, I'm planning to buy a bow, but I absolutely will NOT hunt with it until I am confident of my ability to correctly judge and execute 100% kill shots - even if it takes me years to get there. Sure, even a "perfect shot" usually means the animal will run off (EVERY animal I've ever shot ran off), but a good clean kill shots means its not going to go far.
I won't give you the "Big hole for better blood" lecture, because your .50 cal is obviously plenty big, but there is no way you made a good shot with that round and lost the deer.