I'm not sure if you were the one on this forum or another forum that asked about slugs and slug guns and what reccomendations people had for slugs in their guns and the performance that they should accept as normal for a slug gun 0r those slugs.
Here is my opinion.
You can get just as good of performance out of regular over the counter out of the box Remington Punkin Balls in 2 3/4 inch as you can get out of them high priced SST's that you are now shooting.
An average of 4 inches in any direction from center of a 1 inch bulls eye at 50 yards is probably acceptable in your situation.
Depending on how many shots you took out of your shotgun to try to sight it in before the season started - will tell the rest of the story.
Do you really want to know what you did wrong?
I will tell you!
First off - you only sighted in your gun for 50 yards and then you expected it to shoot 75 yards. You thought that you knew what it would do at 50 yards, but never even bothered to find out what it would do at 75, 100, 125 , 150 yards.
The shots you missed, you shot clear over the deer's back. The shots you made- you shot high.
You call your gun a slug gun, but you never mentioned what make model of shotgun you have, nor the type of scope or scope mounts.
Then you sight your gun in at a shooting range and then you shoot off hard surfaces. That will change your point of aim. Not to mention BUCK FEVER.
You probably were yanking the trigger and it made the gun shoot that much higher.
You mentioned shooting only a couple of shots, which tells me that you work all during the year and then the one day out of the year when you decide to hunt - you think you are Daniel Boone with old Betsy.
You need to spend hours and weeks and months and years of time out in the woods before you make enough mistakes - to learn not to make them mistakes before you can call yourself a real hunter.
I have hunted small game for more than 30 years, and I miss more than I hit. Then when deer season comes around, I use the same type action rifle / shotgun - that way I get used to pushing in the button for the safety and working the action and I can shoot one just as good as the other one.
Use the ammo that you bought for target practice - every weekend if necessary until you get good enough that you can hit aPepsi 12 ounce can 12 out of 12 times. Then when you go hunting deer with Old Betsy - you will hit what you are aiming at. Forget about what the gun did on the sand bags and concentrate on what it does in the field under real life conditions.
Also - you might want to go out and buy a good quality shooting sticks - I useStoney Point Steady Stix, and they will make a good shot out of a fair shot 9 times out of 10. Forget about using a tree branch and droping to one knee as long as you can use the sticks. Practice with the sticks and tell me what comes of it.
The other thing is to get rid of them slugs and buy regular over the counter slugs and practice as much as you can.
http://www.cabelas.com/spodw-1/0005830.shtml
http://www.cabelas.com/spodw-1/0016694.shtml