If you are hitting the animal high you will have little blood no matter which bullet you are using. The description of the wound you describe is evident of that. The reason you started to get blood later is that the chest/lungs began filling up and it was blowing out his nose/mouth. I would guarantee that if you put that SST in the lower 1/3 of the body behind the shoulder(broadside), if the animal doesn't drop right there, he won't go far and you will have a blood trail.
You report is good in that it tells me there is nothing wrong with the SST bullets but more an 'error' in shot placement. Sorry, I don't mean to put you down. And I congratulate you on your success and hope your luck continues. But do yourself a favor a aim a bit lower.