Hey Fast, I just got an USH (SB2-980) with a Nikon slughunter scope which is 3-9-40. You said you scope cost 100 dollars, well I would say it is a cheap scope on a great gun, what can happen is that the scope can't take the recoil of the gun, because slug guns kick more then your average shotgun. That is why, I would get the Nikon Slughunter scope which has a BDC reticule, what it has is four lines on it, first line is 50 yards, second line is 100 yards, third 150 yards, and fourth is 200 yards, what it does is the scope does the compensating for you, so instead of aiming six inches high at 150, all you have to do is take your 150 yard line (in the scope) and aim where you want on the target. This scope is specifically designed for slug guns, the scope cost 250 dollars.
I was afraid that a cheap scope would do what had happened to you, I spent 460 dollars but I am happy with what happened at the range, I got one inch groups at 100 yards, I used Hornady SST'S, they work well for me, I would try different ammo. What was the weather like, because I tried at 150 yards with my USH and got 12 inch patterns, because it was 10 mph to 15 mph winds which throw off the sabots.
I hope this helps
buckshooter